18 Feature Structures

Table of contents

A feature structure is a general purpose data structure which identifies and groups together individual features, each of which associates a name with one or more values. Because of the generality of feature structures, they can be used to represent many different kinds of information, but they are of particular usefulness in the representation of linguistic analyses, especially where such analyses are partial, or underspecified. Feature structures represent the interrelations among various pieces of information, and their instantiation in markup provides a metalanguage for the generic representation of analyses and interpretations. Moreover, this instantiation allows feature values to be of specific types, and for restrictions to be placed on the values for particular features, by means of feature system declarations.82

⚓18.1 Organization of this Chapter

This chapter is organized as follows. Following this introduction, section 18.2 Elementary Feature Structures and the Binary Feature Value introduces the elements fs and f, used to represent feature structures and features respectively, together with the elementary binary feature value. Section 18.3 Other Atomic Feature Values introduces elements for representing other kinds of atomic feature values such as symbolic, numeric, and string values. Section 18.4 Feature Libraries and Feature-Value Libraries introduces the notion of predefined libraries or groups of features or feature values along with methods for referencing their components. Section 18.5 Feature Structures as Complex Feature Values introduces complex values, in particular feature-structures as values, thus enabling feature structures to be recursively defined. Section 18.7 Collections as Complex Feature Values discusses other complex values, in particular values which are collections, organized as sets, bags, and lists. Section 18.8 Feature Value Expressions discusses how the operations of alternation, negation, and collection of feature values may be represented. Section 18.9 Default Values discusses ways of representing underspecified, default, or uncertain values. Section 18.10 Linking Text and Analysis discusses how analyses may be linked to other parts of an encoded text. Section 18.11 Feature System Declaration describes the feature system declaration, a construct which provides for the validation of typed feature structures. Formal definitions for all the elements introduced in this chapter are provided in section 18.12 Formal Definition and Implementation.

⚓18.2 Elementary Feature Structures and the Binary Feature Value

The fundamental elements used to represent a feature structure analysis are f (for feature), which represents a feature-value pair, and fs (for feature structure), which represents a structure made up of such feature-value pairs. The fs element has an optional type attribute which may be used to represent typed feature structures, and may contain any number of f elements. An f element has a required name attribute and an associated value. The value may be simple: that is, a single binary, numeric, symbolic (i.e. taken from a restricted set of legal values), or string value, or a collection of such values, organized in various ways, for example, as a list; or it may be complex, that is, it may itself be a feature structure, thus providing a degree of recursion. Values may be under-specified or defaulted in various ways. These possibilities are all described in more detail in this and the following sections.

Feature and feature-value representations (including feature structure representations) may be embedded directly at any point in an XML document, or they may be collected together in special-purpose feature or feature-value libraries. The components of such libraries may then be referenced from other feature or feature-value representations, using the feats or fVal attribute as appropriate.

We begin by considering the simple case of a feature structure which contains binary-valued features only. The following three XML elements are needed to represent such a feature structure:

  • fs (feature structure) represents a feature structure, that is, a collection of feature-value pairs organized as a structural unit.
    type specifies the type of the feature structure.
    feats (features) references the feature-value specifications making up this feature structure.
  • f (feature) represents a feature value specification, that is, the association of a name with a value of any of several different types.
    name a single word which follows the rules defining a legal XML name (see https://www.w3.org/TR/REC-xml/#dt-name), providing a name for the feature.
    fVal (feature value) references any element which can be used to represent the value of a feature.
  • binary (binary value) represents the value part of a feature-value specification which can contain either of exactly two possible values.

The attributes feats and the fVal are not discussed in this section: they provide an alternative way of indicating the content of an element, as further discussed in section 18.4 Feature Libraries and Feature-Value Libraries.

An fs element containing f elements with binary values can be straightforwardly used to encode the matrices of feature-value specifications for phonetic segments, such as the following for the English segment [s]. 

+--- ---+ | consonantal + | | vocalic - | | voiced - | | anterior + | | coronal + | | continuant + | | strident + | +--- ---+⚓
This representation may be encoded in XML as follows:
<fs type="phonological_segments">
 <f name="consonantal">
  <binary value="true"/>
 </f>
 <f name="vocalic">
  <binary value="false"/>
 </f>
 <f name="voiced">
  <binary value="false"/>
 </f>
 <f name="anterior">
  <binary value="true"/>
 </f>
 <f name="coronal">
  <binary value="true"/>
 </f>
 <f name="continuant">
  <binary value="true"/>
 </f>
 <f name="strident">
  <binary value="true"/>
 </f>
</fs>
Note that fs elements may have an optional type attribute to indicate the kind of feature structure in question, whereas f elements must have a name attribute to indicate the name of the feature. Feature structures need not be typed, but features must be named. Similarly, the fs element may be empty, but the f element must specify its value either directly as content, by means of the fVal attribute, or implicitly by reference to a feature system declaration.

The restriction of specific features to specific types of values (e.g. the restriction of the feature strident to a binary value) requires additional validation, as does any restriction on the features available within a feature structure of a particular type (e.g. whether a feature structure of type phonological segment necessarily contains a feature voiced). Such validation may be carried out at the document level, using special purpose processing, at the schema level using additional validation rules, or at the declarative level, using an additional mechanism such as the feature-system declaration discussed in 18.11 Feature System Declaration.

Although we have used the term binary for this kind of value, and its representation in XML uses values such as true and false (or, equivalently, 1 and 0), it should be noted that such values are not restricted to propositional assertions. As this example shows, this kind of value is intended for use with any binary-valued feature.

⚓18.3 Other Atomic Feature Values

Features may take other kinds of atomic value. In this section, we define elements which may be used to represent: symbolic values, numeric values, and string values. The module defined by this chapter allows for the specification of additional datatypes if necessary, by extending the underlying class model.featureVal.single. If this is done, it is recommended that only the basic W3C datatypes should be used; more complex datatyping should be represented as feature structures.

  • symbol (symbolic value) represents the value part of a feature-value specification which contains one of a finite list of symbols.
    value supplies a symbolic value for the feature, one of a finite list that may be specified in a feature declaration.
  • numeric (numeric value) represents the value part of a feature-value specification which contains a numeric value or range.
  • string (string value) represents the value part of a feature-value specification which contains a string.
The symbol element is used for the value of a feature when that feature can have any of a small, finite set of possible values, representable as character strings. For example, the following might be used to represent the claim that the Latin noun form mensas (tables) has accusative case, feminine gender, and plural number:
<fs>
 <f name="case">
  <symbol value="accusative"/>
 </f>
 <f name="gender">
  <symbol value="feminine"/>
 </f>
 <f name="number">
  <symbol value="plural"/>
 </f>
</fs>
More formally, this representation shows a structure in which three features (case, gender, and number) are used to define morpho-syntactic properties of a word. Each of these features can take one of a small number of values (for example, case can be nominative, genitive, dative, accusative, etc.) and it is therefore appropriate to represent the values taken in this instance as symbol elements. Note that, instead of using a symbolic value for grammatical number, one could have named the feature singular or plural and given it an appropriate binary value, as in the following example:
<fs>
 <f name="case">
  <symbol value="accusative"/>
 </f>
 <f name="gender">
  <symbol value="feminine"/>
 </f>
 <f name="singular">
  <binary value="false"/>
 </f>
</fs>
Whether one uses a binary or symbolic value in situations like this is largely a matter of taste.
The string element is used for the value of a feature when that value is a string drawn from a very large or potentially unbounded set of possible strings of characters, so that it would be impractical or impossible to use the symbol element. The string value is expressed as the content of the string element, rather than as an attribute value. For example, one might encode a street address as follows:
<fs>
 <f name="address">
  <string>3418 East Third Street</string>
 </f>
</fs>
The numeric element is used when the value of a feature is a numeric value, or a range of such values. For example, one might wish to regard the house number and the street name as different features, using an encoding like the following:
<fs>
 <f name="houseNumber">
  <numeric value="3418"/>
 </f>
 <f name="streetName">
  <string>East Third Street</string>
 </f>
</fs>
If the numeric value to be represented falls within a specific range (for example an address that spans several numbers), the max attribute may be used to supply an upper limit:
<fs>
 <f name="houseNumber">
  <numeric value="3418" max="3440"/>
 </f>
 <f name="streetName">
  <string>East Third Street</string>
 </f>
</fs>
It is also possible to specify that the numeric value (or values) represented should (or should not) be truncated. For example, assuming that the daily rainfall in mm is a feature of interest for some address, one might represent this by an encoding like the following:
<fs>
 <f name="dailyRainFall">
  <numeric value="0.0" max="1.3"
   trunc="false"/>
 </f>
</fs>
This represents any of the infinite number of numeric values falling between 0 and 1.3; by contrast
<fs>
 <f name="dailyRainFall">
  <numeric value="0.0" max="1.3"
   trunc="true"/>
 </f>
</fs>
represents only two possible values: 0 and 1.
Some communities of practice, notably those with a strong computer-science bias, prefer to dissociate the information on the value of the given feature from the specification of the data type that this value represents. In such cases, feature values can be provided directly as textual content of f, with the assumption that the data type is specified by the schema. The following is an example taken from ISO 24612, presenting the symbolic values for Active Voice and Simple Present Tense in the untyped form:
<fs>
 <f name="voice">active</f>
 <f name="tense">SimPre</f>
</fs>

As noted above, additional processing is necessary to ensure that appropriate values are supplied for particular features, for example to ensure that the feature singular is not given a value such as <symbol value="feminine"/>. There are two ways of attempting to ensure that only certain combinations of feature names and values are used. First, if the total number of legal combinations is relatively small, one can predefine all of them in a construct known as a feature library, and then reference the combination required using the feats attribute in the enclosing fs element, rather than give it explicitly. This method is suitable in the situation described above, since it requires specifying a total of only ten (5 + 3 + 2) combinations of features and values. Similarly, to ensure that only feature structures containing valid combinations of feature values are used, one can put definitions for all valid feature structures inside a feature value library (so called, since a feature structure may be the value of a feature). A total of 30 feature structures (5 × 3 × 2) is required to enumerate all the possible combinations of individual case, gender and number values in the preceding illustration. We discuss the use of such libraries and their representation in XML further in section 18.4 Feature Libraries and Feature-Value Libraries below.

However, the most general method of attempting to ensure that only legal combinations of feature names and values are used is to provide a feature-system declaration discussed in 18.11 Feature System Declaration.

Whether at the level of feature-system declarations, feature- and feature-value libraries, or individual features, it is possible to align both feature names and their values with standardized external data category repositories such as ISOcat. 83 In the following example, both the feature part_of_speech and its value #commonNoun are aligned with the respective definitions provided by ISO DCR (Data Category Registry), as implemented by ISOcat.
<fs>
<!--...-->
 <f name="part_of_speech"
  dcr:datcat="http://www.isocat.org/datcat/DC-1345" fVal="#commonNoun"
  dcr:valueDatcat="http://www.isocat.org/datcat/DC-1256"/>
<!-- ... -->
</fs>

⚓18.4 Feature Libraries and Feature-Value Libraries

As the examples in the preceding section suggest, the direct encoding of feature structures can be verbose. Moreover, it is often the case that particular feature-value combinations, or feature structures composed of them, are re-used in different analyses. To reduce the size and complexity of the task of encoding feature structures, one may use the feats attribute of the fs element to point to one or more of the feature-value specifications for that element. This indirect method of encoding feature structures presumes that the f elements are assigned unique xml:id values, and are collected together in fLib elements (feature libraries). In the same way, feature values of whatever type can be collected together in fvLib elements (feature-value libraries). If a feature has as its value a feature structure or other value which is predefined in this way, the fVal attribute may be used to point to it, as discussed in the next section. The following elements are used for representing feature libraries and feature-value libraries:

  • fLib (feature library) assembles a library of f (feature) elements.
  • fvLib (feature-value library) assembles a library of reusable feature value elements (including complete feature structures).
For example, suppose a feature library for phonological feature specifications is set up as follows.
<fLib n="phonological features">
 <f xml:id="CNS1" name="consonantal">
  <binary value="true"/>
 </f>
 <f xml:id="CNS0" name="consonantal">
  <binary value="false"/>
 </f>
 <f xml:id="VOC1" name="vocalic">
  <binary value="true"/>
 </f>
 <f xml:id="VOC0" name="vocalic">
  <binary value="false"/>
 </f>
 <f xml:id="VOI1" name="voiced">
  <binary value="true"/>
 </f>
 <f xml:id="VOI0" name="voiced">
  <binary value="false"/>
 </f>
 <f xml:id="ANT1" name="anterior">
  <binary value="true"/>
 </f>
 <f xml:id="ANT0" name="anterior">
  <binary value="false"/>
 </f>
 <f xml:id="COR1" name="coronal">
  <binary value="true"/>
 </f>
 <f xml:id="COR0" name="coronal">
  <binary value="false"/>
 </f>
 <f xml:id="CNT1" name="continuant">
  <binary value="true"/>
 </f>
 <f xml:id="CNT0" name="continuant">
  <binary value="false"/>
 </f>
 <f xml:id="STR1" name="strident">
  <binary value="true"/>
 </f>
 <f xml:id="STR0" name="strident">
  <binary value="false"/>
 </f>
<!-- ... -->
</fLib>
Then the feature structures that represent the analysis of the phonological segments (phonemes) /t/, /d/, /s/, and /z/ may be defined as follows.
<fs feats="#CNS1 #VOC0 #VOI0 #ANT1 #COR1 #CNT0 #STR0"/>
<fs feats="#CNS1 #VOC0 #VOI1 #ANT1 #COR1 #CNT0 #STR0"/>
<fs feats="#CNS1 #VOC0 #VOI0 #ANT1 #COR1 #CNT1 #STR1"/>
<fs feats="#CNS1 #VOC0 #VOI1 #ANT1 #COR1 #CNT1 #STR1"/>
The preceding are but four of the 128 logically possible fully specified phonological segments using the seven binary features listed in the feature library. Presumably not all combinations of features correspond to phonological segments (there are no strident vowels, for example). The legal combinations, however, can be collected together, each one represented as an identifiable fs element within a feature-value library, as in the following example:
<fvLib xml:id="fsl1"
 n="phonological segment definitions">
<!-- ... -->
 <fs xml:id="T.DF"
  feats="#CNS1 #VOC0 #VOI0 #ANT1 #COR1 #CNT0 #STR0"/>
 <fs xml:id="D.DF"
  feats="#CNS1 #VOC0 #VOI1 #ANT1 #COR1 #CNT0 #STR0"/>
 <fs xml:id="S.DF"
  feats="#CNS1 #VOC0 #VOI0 #ANT1 #COR1 #CNT1 #STR1"/>
 <fs xml:id="Z.DF"
  feats="#CNS1 #VOC0 #VOI1 #ANT1 #COR1 #CNT1 #STR1"/>
<!-- ... -->
</fvLib>
Once defined, these feature structure values can also be reused. Other f elements may invoke them by reference, using the fVal attribute; for example, one might use them in a feature value pair such as:
<f name="dental-fricative" fVal="#T.DF"/>
rather than expanding the hierarchy of the component phonological features explicitly.

Feature structures stored in this way may also be associated with the text which they are intended to annotate, either by a link from the text (for example, using the TEI global ana attribute), or by means of stand-off annotation techniques (for example, using the TEI link element): see further section 18.10 Linking Text and Analysis below.

Note that when features or feature structures are linked to in this way, the result is effectively a copy of the item linked to into the place from which it is linked. This form of linking should be distinguished from the phenomenon of structure-sharing, where it is desired to indicate that some part of an annotation structure appears simultaneously in two or more places within the structure. This kind of annotation should be represented using the vLabel element, as discussed in 18.6 Re-entrant Feature Structures below.

⚓18.5 Feature Structures as Complex Feature Values

Features may have complex values as well as atomic ones; the simplest such complex value is represented by supplying an fs element as the content of an f element, or (equivalently) by supplying the identifier of an fs element as the value for the fVal attribute on the f element. Structures may be nested as deeply as appropriate, using this mechanism. For example, an fs element may contain or point to an f element, which may contain or point to an fs element, which may contain or point to an f element, and so on.

To illustrate the use of complex values, consider the following simple model of a word, as a structure combining surface form information, a syntactic category, and semantic information. Each word analysis is represented as a <fs type='word'> element, containing three features named surface, syntax, and semantics. The first of these has an atomic string value, but the other two have complex values, represented as nested feature structures of types category and act respectively:
<fs type="word">
 <f name="surface">
  <string>love</string>
 </f>
 <f name="syntax">
  <fs type="category">
   <f name="pos">
    <symbol value="verb"/>
   </f>
   <f name="val">
    <symbol value="transitive"/>
   </f>
  </fs>
 </f>
 <f name="semantics">
  <fs type="act">
   <f name="rel">
    <symbol value="LOVE"/>
   </f>
  </fs>
 </f>
</fs>
This analysis does not tell us much about the meaning of the symbols verb or transitive. It might be preferable to replace these atomic feature values by feature structures. Suppose therefore that we maintain a feature-value library for each of the major syntactic categories (N, V, ADJ, PREP):
<fvLib n="Major category definitions">
<!-- ... -->
 <fs xml:id="N" type="noun">
<!-- noun features defined here -->
 </fs>
 <fs xml:id="V" type="verb">
<!-- verb features defined here -->
 </fs>
</fvLib>
This library allows us to use shortcut codes (N, V, etc.) to reference a complete definition for the corresponding feature structure. Each definition may be explicitly contained within the fs element, as a number of f elements. Alternatively, the relevant features may be referenced by their identifiers, supplied as the value of the feats attribute, as in these examples:
<!-- ... --> <fs xml:id="ADJ" type="adjective" feats="#F1 #F2"/> <fs xml:id="PREP" type="preposition" feats="#F1 #F3"/> <!-- ... -->
This ability to re-use feature definitions within multiple feature structure definitions is an essential simplification in any realistic example. In this case, we assume the existence of a feature library containing specifications for the basic feature categories like the following:
<fLib n="categorial features">
 <f xml:id="NN-1" name="nominal">
  <binary value="true"/>
 </f>
 <f xml:id="NN-0" name="nominal">
  <binary value="false"/>
 </f>
 <f xml:id="VV-1" name="verbal">
  <binary value="true"/>
 </f>
 <f xml:id="VV-0" name="verbal">
  <binary value="false"/>
 </f>
<!-- ... -->
</fLib>
With such libraries in place, and assuming the availability of similarly predefined feature structures for transitivity and semantics, the preceding example could be considerably simplified:
<fs type="word">
 <f name="surface">
  <string>love</string>
 </f>
 <f name="syntax">
  <fs type="category">
   <f name="pos" fVal="#V"/>
   <f name="val" fVal="#TRNS"/>
  </fs>
 </f>
 <f name="semantics">
  <fs type="act">
   <f name="rel" fVal="#LOVE"/>
  </fs>
 </f>
</fs>

Although in principle the fVal attribute could point to any kind of feature value, its use is not recommended for simple atomic values.

⚓18.6 Re-entrant Feature Structures

Sometimes the same feature value is required at multiple places within a feature structure, in particular where the value is only partially specified at one or more places. The vLabel element is provided as a means of labelling each such re-entrancy point:

  • vLabel (value label) represents the value part of a feature-value specification which appears at more than one point in a feature structure.
For example, suppose one wishes to represent noun-verb agreement as a single feature structure. Within the representation, the feature indicating (say) number appears more than once. To represent the fact that each occurrence is another appearance of the same feature (rather than a copy) one could use an encoding like the following:
<fs xml:id="NVA">
 <f name="nominal">
  <fs>
   <f name="nm-num">
    <vLabel name="L1">
     <symbol value="singular"/>
    </vLabel>
   </f>
<!-- other nominal features -->
  </fs>
 </f>
 <f name="verbal">
  <fs>
   <f name="vb-num">
    <vLabel name="L1"/>
   </f>
  </fs>
<!-- other verbal features -->
 </f>
</fs>

In the above encoding, the features named vb-num and nm-num exhibit structure sharing. Their values, given as vLabel elements, are understood to be references to the same point in the feature structure, which is labelled by their name attribute.

The scope of the names used to label re-entrancy points is that of the outermost fs element in which they appear. When a feature structure is imported from a feature value library, or referenced from elsewhere (for example by using the fVal attribute) the names of any sharing points it may contain are implicitly prefixed by the identifier used for the imported feature structure, to avoid name clashes. Thus, if some other feature structure were to reference the fs element given in the example above, for example in this way:
<f name="class" fVal="#NVA"/>
then the labelled points in the example would be interpreted as if they had the name NVAL1.

⚓18.7 Collections as Complex Feature Values

Complex feature values need not always be represented as feature structures. Multiple values may also be organized as sets, bags or multisets, or lists of atomic values of any type. The vColl element is provided to represent such cases:

  • vColl (collection of values) represents the value part of a feature-value specification which contains multiple values organized as a set, bag, or list.

A feature whose value is regarded as a set, bag, or list may have any positive number of values as its content, or none at all, (thus allowing for representation of the empty set, bag, or list). The items in a list are ordered, and need not be distinct. The items in a set are not ordered, and must be distinct. The items in a bag are neither ordered nor distinct. Sets and bags are thus distinguished from lists in that the order in which the values are specified does not matter for the former, but does matter for the latter, while sets are distinguished from bags and lists in that repetitions of values do not count for the former but do count for the latter.

If no value is specified for the org attribute, the assumption is that the vColl defines a list of values. If the vColl element is empty, the assumption is that it represents the null list, set, or bag.

To illustrate the use of the org attribute, suppose that a feature structure analysis is used to represent a genealogical tree, with the information about each individual treated as a single feature structure, like this:
<fs xml:id="p027" type="person">
 <f name="forenames">
  <vColl>
   <string>Daniel</string>
   <string>Edouard</string>
  </vColl>
 </f>
 <f name="mother" fVal="#p002"/>
 <f name="father" fVal="#p009"/>
 <f name="birthDate">
  <fs type="date" feats="#y1988 #m04 #d17"/>
 </f>
 <f name="birthPlace" fVal="#austintx"/>
 <f name="siblings">
  <vColl org="set">
   <fs copyOf="#pnb005"/>
   <fs copyOf="#prb001"/>
  </vColl>
 </f>
</fs>

In this example, the vColl element is first used to supply a list of ‘name’ feature values, which together constitute the ‘forenames’ feature. Other features are defined by reference to values which we assume are held in some external feature value library (not shown here). For example, the vColl element is used a second time to indicate that the persons's siblings should be regarded as constituting a set rather than a list. Each sibling is represented by a feature structure: in this example, each feature structure is a copy of one specified in the feature value library.

If a specific feature contains only a single feature structure as its value, the component features of which are organized as a set, bag, or list, it may be more convenient to represent the value as a vColl rather than as an fs. For example, consider the following encoding of the English verb form sinks, which contains an agreement feature whose value is a feature structure which contains person and number features with symbolic values.
<fs type="word">
 <f name="category">
  <symbol value="verb"/>
 </f>
 <f name="tense">
  <symbol value="present"/>
 </f>
 <f name="agreement">
  <fs>
   <f name="person">
    <symbol value="third"/>
   </f>
   <f name="number">
    <symbol value="singular"/>
   </f>
  </fs>
 </f>
</fs>
If the names of the features contained within the agreement feature structure are of no particular significance, the following simpler representation may be used:
<fs type="word">
 <f name="category">
  <symbol value="verb"/>
 </f>
 <f name="tense">
  <symbol value="present"/>
 </f>
 <f name="agreement">
  <vColl org="set">
   <symbol value="third"/>
   <symbol value="singular"/>
  </vColl>
 </f>
</fs>
The vColl element is also useful in cases where an analysis has several components. In the following example, the French word auxquels has a two-part analysis, represented as a list of two values. The first specifies that the word contains a preposition; the second that it contains a masculine plural relative pronoun:
<fs>
 <f name="lex">
  <symbol value="auxquels"/>
 </f>
 <f name="maf">
  <vColl org="list">
   <fs>
    <f name="cat">
     <symbol value="prep"/>
    </f>
   </fs>
   <fs>
    <f name="cat">
     <symbol value="pronoun"/>
    </f>
    <f name="kind">
     <symbol value="rel"/>
    </f>
    <f name="num">
     <symbol value="pl"/>
    </f>
    <f name="gender">
     <symbol value="masc"/>
    </f>
   </fs>
  </vColl>
 </f>
</fs>

The set, bag, or list which has no members is known as the null (or empty) set, bag, or list. A vColl element with no content and with no value for its feats attribute is interpreted as referring to the null set, bag, or list, depending on the value of its org attribute.

If, for example, the individual described by the feature structure with identifier p027 (above) had no siblings, we might specify the siblings feature as follows.
<f name="siblings">
 <vColl org="set"/>
</f>

A vColl element may also collect together one or more other vColl elements, if, for example one of the members of a set is itself a set, or if two lists are concatenated together. Note that such collections pay no attention to the contents of the nested vColl elements: if it is desired to produce the union of two sets, the vMerge element discussed below should be used to make a new collection from the two sets.

⚓18.8 Feature Value Expressions

It is sometimes desirable to express the value of a feature as the result of an operation over some other value (for example, as ‘not green’, or as ‘male or female’, or as the concatenation of two collections). Three special purpose elements are provided to represent disjunctive alternation, negation, and collection of values:

  • vAlt (value alternation) represents the value part of a feature-value specification which contains a set of values, only one of which can be valid.
  • vNot (value negation) represents a feature value which is the negation of its content.
  • vMerge (merged collection of values) represents a feature value which is the result of merging together the feature values contained by its children, using the organization specified by the org attribute.

⚓18.8.1 Alternation

The vAlt element can be used wherever a feature value can appear. It contains two or more feature values, any one of which is to be understood as the value required. Suppose, for example, that we are using a feature system to describe residential property, using such features as number.of.bathrooms. In a particular case, we might wish to represent uncertainty as to whether a house has two or three bathrooms. As we have already shown, one simple way to represent this would be with a numeric maximum:
<f name="number.of.bathrooms">
 <numeric value="2" max="3"/>
</f>
A more general way would be to represent the alternation explicitly, in this way:
<f name="number.of.bathrooms">
 <vAlt>
  <numeric value="2"/>
  <numeric value="3"/>
 </vAlt>
</f>
The vAlt element represents alternation over feature values, not feature-value pairs. If therefore the uncertainty relates to two or more feature value specifications, each must be represented as a feature structure, since a feature structure can always appear where a value is required. For example, suppose that it is uncertain as to whether the house being described has two bathrooms or two bedrooms, a structure like the following may be used:
<f name="rooms">
 <vAlt>
  <fs>
   <f name="number.of.bathrooms">
    <numeric value="2"/>
   </f>
  </fs>
  <fs>
   <f name="number.of.bedrooms">
    <numeric value="2"/>
   </f>
  </fs>
 </vAlt>
</f>
Note that alternation is always regarded as exclusive: in the case above, the implication is that having two bathrooms excludes the possibility of having two bedrooms and vice versa. If inclusive alternation is required, a vColl element may be included in the alternation as follows:
<f name="rooms">
 <vAlt>
  <fs>
   <f name="number.of.bathrooms">
    <numeric value="2"/>
   </f>
  </fs>
  <fs>
   <f name="number.of.bedrooms">
    <numeric value="2"/>
   </f>
  </fs>
  <vColl>
   <fs>
    <f name="number.of.bathrooms">
     <numeric value="2"/>
    </f>
   </fs>
   <fs>
    <f name="number.of.bedrooms">
     <numeric value="2"/>
    </f>
   </fs>
  </vColl>
 </vAlt>
</f>
This analysis indicates that the property may have two bathrooms, two bedrooms, or both two bathrooms and two bedrooms.
As the previous example shows, the vAlt element can also be used to indicate alternations among values of features organized as sets, bags or lists. Suppose we use a feature selling.points to describe items that are mentioned to enhance a property's sales value, such as whether it has a pool or a good view. Now suppose for a particular listing, the selling points include an alarm system and a good view, and either a pool or a jacuzzi (but not both). This situation could be represented, using the vAlt element, as follows.
<fs type="real_estate_listing">
 <f name="selling.points">
  <vColl org="set">
   <string>alarm system</string>
   <string>good view</string>
   <vAlt>
    <string>pool</string>
    <string>jacuzzi</string>
   </vAlt>
  </vColl>
 </f>
</fs>
Now suppose the situation is like the preceding except that one is also uncertain whether the property has an alarm system or a good view. This can be represented as follows.
<fs type="real_estate_listing">
 <f name="selling.points">
  <vColl org="set">
   <vAlt>
    <string>alarm system</string>
    <string>good view</string>
   </vAlt>
   <vAlt>
    <string>pool</string>
    <string>jacuzzi</string>
   </vAlt>
  </vColl>
 </f>
</fs>

If a large number of ambiguities or uncertainties need to be represented, involving a relatively small number of features and values, it is recommended that a stand-off technique, for example using the general-purpose alt element discussed in section 16.8 Alternation be used, rather than the special-purpose vAlt element.

⚓18.8.2 Negation

The vNot element can be used wherever a feature value can appear. It contains any feature value and returns the complement of its contents. For example, the feature number.of.bathrooms in the following example has any whole numeric value other than 2:
<f name="number.of.bathrooms">
 <vNot>
  <numeric value="2"/>
 </vNot>
</f>
Strictly speaking, the effect of the vNot element is to provide the complement of the feature values it contains, rather than their negation. If a feature system declaration is available which defines the possible values for the associated feature, then it is possible to say more about the negated value. For example, suppose that the available values for the feature case are declared to be nominative, genitive, dative, or accusative, whether in a TEI feature system declaration or by some other means. Then the following two specifications are equivalent:
(i) <f name="case">
 <vNot>
  <symbol value="genitive"/>
 </vNot>
</f>
(ii)
<f name="case">
 <vAlt>
  <symbol value="nominative"/>
  <symbol value="dative"/>
  <symbol value="accusative"/>
 </vAlt>
</f>

If however no such system declaration is available, all that one can say about a feature specified via negation is that its value is something other than the negated value.

Negation is always applied to a feature value, rather than to a feature-value pair. The negation of an atomic value is the set of all other values which are possible for the feature.

Any kind of value can be negated, including collections (represented by a vColl elements) or feature structures (represented by fs elements). The negation of any complex value is understood to be the set of values which cannot be unified with it. Thus, for example, the negation of the feature structure F is understood to be the set of feature structures which are not unifiable with F. In the absence of a constraint mechanism such as the Feature System Declaration, the negation of a collection is anything that is not unifiable with it, including collections of different types and atomic values. It will generally be more useful to require that the organization of the negated value be the same as that of the original value, for example that a negated set is understood to mean the set which is a complement of the set, but such a requirement cannot be enforced in the absence of a constraint mechanism.

⚓18.8.3 Collection of Values

The vMerge element can be used wherever a feature value can appear. It contains two or more feature values, all of which are to be collected together. The organization of the resulting collection is specified by the value of the org attribute, which need not necessarily be the same as that of its constituent values if these are collections. For example, one can change a list to a set, or vice versa.

As an example, suppose that we wish to represent the range of possible values for a feature ‘genders’ used to describe some language. It would be natural to represent the possible values as a set, using the vColl element as in the following example:
<fs>
 <f name="genders">
  <vColl org="set">
   <symbol value="masculine"/>
   <symbol value="feminine"/>
  </vColl>
 </f>
</fs>
Suppose however that we discover for some language it is necessary to add a new possible value, and to treat the value of the feature as a list rather than as a set. The vMerge element can be used to achieve this:
<fs>
 <f name="genders">
  <vMerge org="list">
   <vColl org="set">
    <symbol value="masculine"/>
    <symbol value="feminine"/>
   </vColl>
   <symbol value="neuter"/>
  </vMerge>
 </f>
</fs>

⚓18.9 Default Values

The value of a feature may be underspecified in a number of different ways. It may be null, unknown, or uncertain with respect to a range of known possibilities, as well as being defined as a negation or an alternation. As previously noted, the specification of the range of known possibilities for a given feature is not part of the current specification: in the TEI scheme, this information is conveyed by the feature system declaration. Using this, or some other system, we might specify (for example) that the range of values for an element includes symbols for masculine, feminine, and neuter, and that the default value is neuter. With such definitions available to us, it becomes possible to say that some feature takes the default value, or some unspecified value from the list. The following special element is provided for this purpose:

  • default (default feature value) represents the value part of a feature-value specification which contains a defaulted value.
The value of an empty f element which also lacks an fVal attribute is understood to be the most general case, i.e. any of the available values. Thus, assuming the feature system defined above, the following two representations are equivalent.
<f name="gender"/>
<f name="gender">
 <vAlt>
  <symbol value="feminine"/>
  <symbol value="masculine"/>
  <symbol value="neuter"/>
 </vAlt>
</f>
If, however, the value is explicitly stated to be the default one, using the default element, then the following two representations are equivalent:
<f name="gender">
 <default/>
</f>
<f name="gender">
 <symbol value="neuter"/>
</f>
Similarly, if the value is stated to be the negation of the default, then the following two representations are equivalent:
<f name="gender">
 <vNot>
  <default/>
 </vNot>
</f>
<f name="gender">
 <vAlt>
  <symbol value="feminine"/>
  <symbol value="masculine"/>
 </vAlt>
</f>

⚓18.11 Feature System Declaration

The Feature System Declaration (FSD) is intended for use in conjunction with a TEI-conforming text that makes use of fs (that is, feature structure) elements. The FSD serves three purposes:

  • the encoder can list all of the feature names and feature values and give a prose description as to what each represents.
  • the encoder can define what it means to be a well-formed feature structure, and define constraints which may be used to determine whether a particular feature structure is valid relative to a given theory stated in typed feature logic. These may involve constraints on the range of a feature value, constraints on what features are valid within certain types of feature structures, or constraints that prevent the co-occurrence of certain feature-value pairs.
  • the encoder can define the intended interpretation of underspecified feature structures. This involves defining default values (whether literal or computed) for missing features.

The scheme described in this chapter may be used to document any feature structure system, but is primarily intended for use with the feature structure representation defined by the ISO 24610-1:2006 standard, which corresponds with the recommendations presented in these Guidelines, 18 Feature Structures. This chapter relies upon, but does not reproduce, formal definitions and descriptions presented more thoroughly in the ISO standard, which should be consulted in case of ambiguity or uncertainty.

The FSD serves an important function in documenting precisely what the encoder intended by the system of feature structure markup used in an XML-encoded text. The FSD is also an important resource which standardizes the rules of inference used by software to validate the feature structure markup in a text, and to infer the full interpretation of underspecified feature structures.

The reader should be aware the terminology used in this document does not always closely follow conventional practice in formal logic, and may also diverge from practice in some linguistic applications of typed feature structures. In particular, the term ‘interpretation’ when applied to a feature structure is not an interpretation in the model-theoretic sense, but is instead a minimally informative (or equivalently, most general) extension of that feature structure that is consistent with a set of constraints declared by an FSD. In linguistic application, such a system of constraints is the principal means by which the grammar of some natural language is expressed. There is a great deal of disagreement as to what, if any, model-theoretic interpretation feature structures have in such applications, but the status of this formal kind of interpretation is not germane to the present document. Similarly, the term ‘valid’ is used here as elsewhere in these Guidelines to identify the syntactic state of well-formedness in the sense defined by the logic of typed feature structures itself, as distinct from and in addition to the ‘well-formedness’ that pertains at the level of this encoding standard. No appeal to any notion from formal semantics should be inferred.

We begin by describing how an encoded text is associated with one or more feature system declarations. The second, third, and fourth sections describe the overall structure of a feature system declaration and give details of how to encode its components. The final section offers a full example; fuller discussion of the reasoning behind FSDs and another complete example are provided in Langendoen and Simons (1995).

⚓18.11.1 Linking a TEI Text to Feature System Declarations

In order for application software to use feature system declarations to aid in the automatic interpretation of encoded texts, or even for human readers to find the appropriate declarations which document the feature system used in markup, there must be a formal link from the encoded texts to the declarations. However, the schema which declares the syntax of the Feature System itself should be kept distinct from the feature structure schema, which is an application of that system.

A document containing typed feature structures may simply include a feature system declaration documenting those feature structures. A more usual scenario, however, is that the same feature system declaration (or parts of it) will be shared by many documents. In either case, an fsDecl element for each distinct type of feature structure used must be provided and associated with the type, which is the value used within each feature structure for its type attribute.

When the module defined in this chapter is included in an XML schema, the following elements become available via the model.fsdDeclPart class:

  • fsdDecl (feature system declaration) provides a feature system declaration comprising one or more feature structure declarations or feature structure declaration links.
  • model.fsdDeclPart groups elements which can occur as direct children of fsdDecl.
    fLib (feature library) assembles a library of f (feature) elements.
    fsDecl (feature structure declaration) declares one type of feature structure.
    fsdLink (feature structure declaration link) associates the name of a typed feature structure with a feature structure declaration for it.
    fvLib (feature-value library) assembles a library of reusable feature value elements (including complete feature structures).

The fsdDecl element serves as a wrapper for declaring feature systems and may be supplied either within the header of a standard TEI document, or as a standalone document in its own right. It contains one or more fsdLink or fsDecl elements and may hold several fLib or fvLib as well.

For example, suppose that a document doc.xml contains feature structures of two types: gpsg and lex. We might simply embed an fsDecl element for each within the header attached to the document as follows:
<TEI xmlns="http://www.tei-c.org/ns/1.0">
 <teiHeader>
  <fileDesc>
<!-- example -->
  </fileDesc>
  <encodingDesc>
<!-- ... -->
   <fsdDecl>
    <fsDecl type="gpsg">
<!-- information about this type -->
    </fsDecl>
    <fsDecl type="lex">
<!-- information about this type -->
    </fsDecl>
   </fsdDecl>
<!-- ... -->
  </encodingDesc>
 </teiHeader>
 <text>
  <body>
<!-- ... -->
   <fs type="lex">
<!-- an instance of the typed feature structure "lex" -->
   </fs>
<!-- ... -->
  </body>
 </text>
</TEI>

In this case there is an implicit link between the fs element and the corresponding fsDecl element because they share the same value for their type attribute and appear within the same document. This is a short cut for the more general case which requires a more explicit link provided by means of the fsdLink element, as demonstrated below.

Now suppose that we wish to create a second document which includes feature structures of the same type. Rather than duplicate the corresponding declarations, we will need to provide a means of pointing to them from this second document. The easiest84 way of accomplishing this is to add an XML identifier to each fsDecl element in example.xml:

<!-- ... --><fsdDecl>
 <fsDecl type="gpsg" xml:id="GPSG">
<!-- information about this type -->
 </fsDecl>
 <fsDecl type="lex" xml:id="LEX">
<!-- information about this type -->
 </fsDecl>
</fsdDecl>
(Although in this case the XML identifier is simply an uppercase version of the type name, there is no necessary connection between the two names. The only requirement is that the XML identifier conform to the standards required for identifiers, and that it be unique within the document containing it.)
In the fsdDecl for the second document, we can now include pointers to the fsDecl elements in the first:
<TEI xmlns="http://www.tei-c.org/ns/1.0">
 <teiHeader>
  <fileDesc>
<!-- doc2 -->
  </fileDesc>
  <encodingDesc>
<!-- ... -->
   <fsdDecl>
    <fsdLink type="gpsg"
     target="example.xml#GPSG"/>
    <fsdLink type="lexx"
     target="example.xml#LEX"/>
   </fsdDecl>
<!-- ... -->
  </encodingDesc>
 </teiHeader>
 <text>
  <body>
<!-- ... -->
   <fs type="lexx">
<!-- an instance of the typed feature structure "lex" -->
   </fs>
<!-- ... -->
  </body>
 </text>
</TEI>
Note that in doc2.xml there is no requirement for the local name for a given type of feature structures to be the same as that used by example.xml. We assume in this encoding that the type called lexx in doc2.xml is declared as having identical constraints and other properties to those declared for the type called lex in example.xml.

An fsdDecl may be given, as above, within the encoding description of the teiHeader element of a TEI document containing typed feature structures. Alternatively, it may appear independently of any feature structures, as a document in its own right with its own teiHeader. These options are both possible because the element is a member of both the model.encodingDescPart class and the model.resource class.

The current recommendations provide no way of enforcing uniqueness of the type values among fsdDecl elements, nor of requiring that every type value specified on an fs element be also declared on an fsdDecl element. Encoders requiring such constraints (which might have some obvious utility in assisting the consistency and accuracy of tagging) are recommended to develop tools to enforce them, using such mechanisms as Schematron assertions.

⚓18.11.2 The Overall Structure of a Feature System Declaration

A feature system declaration contains one or more feature structure declarations, each of which has up to three parts: an optional description (which gives a prose comment on what that type of feature structure encodes), an obligatory set of feature declarations (which specify range constraints and default values for the features in that type of structure), and optional feature structure constraints (which specify co-occurrence restrictions on feature values).

  • fsDescr (feature system description (in FSD)) describes in prose what is represented by the type of feature structure declared in the enclosing fsDecl.
  • fDecl (feature declaration) declares a single feature, specifying its name, organization, range of allowed values, and optionally its default value.
  • fsConstraints (feature-structure constraints) specifies constraints on the content of valid feature structures.

Feature declarations and feature structure constraints are described in the next two sections. Note that the specification of similar fsDecl elements can be simplified by devising an inheritance hierarchy for the feature structure types. Each fsDecl element may name one or more ‘basetypes’ from which it inherits feature declarations and constraints (these are often called ‘supertypes’). For instance, suppose that <fsDecl type="Basic"> contains <fDecl name="One"> and <fDecl name="Two">, and that <fsDecl type="Derived" baseTypes="Basic"> contains just <fDecl name="Three">. Then any instance of <fs type="Derived"> must include all three features. This is because <fsDecl type="Derived"> inherits the two feature declarations from <fsDecl type="Basic"> when it specifies a base type of Basic.

The following sample shows the overall structure of a complete feature structure declaration:
<fsDecl type="SomeName">
 <fsDescr>Describes what this type of fs represents</fsDescr>
 <fDecl name="featureOne">
<!-- The declaration for featureOne -->
 </fDecl>
 <fDecl name="featureTwo">
<!-- The declaration for featureTwo -->
 </fDecl>
 <fsConstraints>
<!-- The feature structure constraints go here -->
 </fsConstraints>
</fsDecl>

The attribute baseTypes gives the name of one or more types from which this type inherits feature specifications and constraints; if this type includes a feature specification with the same name as one inherited from any of the types specified by this attribute, or if more than one specification of the same name is inherited, then the possible values of that feature is determined by unification. Similarly, the set of constraints applicable is derived by conjoining those specified explicitly within this element with those implied by the baseTypes attribute. When no base type is specified, no feature specification or constraint is inherited.

Although the present standard does provide for default feature values, feature inheritance is defined to be monotonic.

The process of combining constraints may result in a contradiction, for example if two specifications for the same feature specify disjoint ranges of values, and at least one such specification is mandatory. In such a case, there is no valid feature structure of the type being defined.

Every type specified by baseTypes must be a single word which is a legal XML name; for example, they cannot include whitespace or begin with digits. Multiple base types are separated with spaces, e.g. <fsDecl type="Sub" baseTypes="Super1 Super2">.

⚓18.11.3 Feature Declarations

Each feature is declared in an fDecl element whose name attribute identifies the feature being declared; this matches the name attribute of the f elements it declares. An fDecl has three parts: an optional prose description (which should explain what the feature and its values represent), an obligatory range specification (which declares what values the feature is allowed to have), and an optional default specification (which declares what default value should be supplied when the named feature does not appear in an fs). If, in a feature structure, a feature:

  • is not optional (i.e., is obligatory),
  • has no value provided, or the value default is provided (see ISO 24610-1, Subclause 5.10, Default Values, and
  • either has no default specified, or has conditional defaults, none of the conditions on which is met,

then the value of this feature in the feature structure's most general valid extension is the most general value provided in its vRange, in the case of a unit organization, or the singleton set, bag, or list containing that element, in the case of a complex organization. If the feature:

  • is optional,
  • has no value provided, or the value default is provided, and
  • either has a default specified, or has conditional defaults, one of the conditions on which is met,

then this feature does have a value in the feature structure's most general valid extension when it exists, namely the default value that pertains.

It is possible that a feature structure will not have a valid extension because the default value that pertains to a feature is not consistent with that feature's declared range. Additional tools are required for the enforcement of such criteria.

The following elements are used in feature system declarations:

  • fDecl (feature declaration) declares a single feature, specifying its name, organization, range of allowed values, and optionally its default value.
    name a single word which follows the rules defining a legal XML name (see https://www.w3.org/TR/REC-xml/#dt-name), indicating the name of the feature being declared; matches the name attribute of f elements in the text.
    optional indicates whether or not the value of this feature may be present.
  • fDescr (feature description (in FSD)) describes in prose what is represented by the feature being declared and its values.
  • vRange (value range) defines the range of allowed values for a feature, in the form of an fs, vAlt, or primitive value; for the value of an f to be valid, it must be subsumed by the specified range; if the f contains multiple values (as sanctioned by the org attribute), then each value must be subsumed by the vRange.
  • vDefault (value default) declares the default value to be supplied when a feature structure does not contain an instance of f for this name; if unconditional, it is specified as one (or, depending on the value of the org attribute of the enclosing fDecl) more fs elements or primitive values; if conditional, it is specified as one or more if elements; if no default is specified, or no condition matches, the value none is assumed.
  • if defines a conditional default value for a feature; the condition is specified as a feature structure, and is met if it subsumes the feature structure in the text for which a default value is sought.
  • then separates the condition from the default in an if, or the antecedent and the consequent in a cond element.

The logic for validating feature values and for matching the conditions for supplying default values is based on the operation of subsumption. Subsumption is a standard operation in feature-structure-based formalisms. Informally, a feature structure FS subsumes all feature structures that are at least as informative as itself; that is, all feature structures that specify all of the feature values that FS does with values that are subsumed by the values that FS has, and that have all of the re-entrancies (see 18.6 Re-entrant Feature Structures) that FS does. (Carpenter (1992); see also Pereira (1987) and Shieber (1986)) A more formal definition is provided in ISO 24610-1:2006 .

Following the spirit of the informal definition above, we can extend subsumption in a straightforward way to cover alternation, negation, special primitive values, and the use of attributes in the markup. For instance, a vAlt containing the value v subsumes v. The negation of a value v (represented by means of the vNot element discussed in section 18.8.2 Negation) subsumes any value that is not v; for example <vNot><numeric value='0'/></vNot> subsumes any numeric value other than zero. The value <fs type="X"/> subsumes any feature structure of type X, even if it is not valid.

As an example of feature declarations, consider the following extract from Gazdar et al.'s Generalized Phrase Structure Grammar. In the appendix to their book, they propose a feature system for English of which this is just a sampling:
feature    value range INV        {+, -} CONJ       {and, both, but, either, neither, nor, or, NIL} COMP       {for, that, whether, if, NIL} AGR        CAT PFORM      {to, by, for, ...}
Feature specification defaults FSD 1:  [-INV] FSD 2:  ~[CONJ] FSD 9:  [INF, +SUBJ] --> [COMP for]
The INV feature, which encodes whether or not a sentence is inverted, allows only the values plus (+) and minus (-). If the feature is not specified, then the default rule (FSD 1 above) says that a value of minus is always assumed. The feature declaration for this feature would be encoded as follows:
<fDecl name="INV">
 <fDescr>inverted sentence</fDescr>
 <vRange>
  <vAlt>
   <binary value="true"/>
   <binary value="false"/>
  </vAlt>
 </vRange>
 <vDefault>
  <binary value="false"/>
 </vDefault>
</fDecl>

The value range is specified as an alternation (more precisely, an exclusive disjunction), which can be represented by the binary feature value. That is, the value must be either true or false, but cannot be both or neither.

The CONJ feature indicates the surface form of the conjunction used in a construction. The ~ in the default rule (see FSD 2 above) represents negation. This means that by default the feature is not applicable, in other words, no conjunction is taking place. Note that CONJ not being present is distinct from CONJ being present but having the NIL value allowed in the value range. In their analysis, NIL means that the phenomenon of conjunction is taking place but there is no explicit conjunction in the surface form of the sentence. The feature declaration for this feature would be encoded as follows:
<fDecl name="CONJ">
 <fDescr>surface form of the conjunction</fDescr>
 <vRange>
  <vAlt>
   <symbol value="and"/>
   <symbol value="both"/>
   <symbol value="but"/>
   <symbol value="either"/>
   <symbol value="neither"/>
   <symbol value="nor"/>
   <symbol value="or"/>
   <symbol value="NIL"/>
  </vAlt>
 </vRange>
 <vDefault>
  <binary value="false"/>
 </vDefault>
</fDecl>
Note that the vDefault is not strictly necessary in this case, since the binary value of false only serves to convey the information that the feature has no other legitimate value.
The COMP feature indicates the surface form of the complementizer used in a construction. In value range, it is analogous to CONJ. However, its default rule (see FSD 9 above) is conditional. It says that if the verb form is infinitival (the VFORM feature is not mentioned in the rule since it is the only feature that can take INF as a value), and the construction has a subject, then a for complement must be used. For instance, to make John the subject of the infinitive in It is necessary to go, a for complement must be used; that is, It is necessary for John to go. The feature declaration for this feature would be encoded as follows:
<fDecl name="COMP">
 <fDescr>surface form of the complementizer</fDescr>
 <vRange>
  <vAlt>
   <symbol value="for"/>
   <symbol value="that"/>
   <symbol value="whether"/>
   <symbol value="if"/>
   <symbol value="NIL"/>
  </vAlt>
 </vRange>
 <vDefault>
  <if>
   <fs>
    <f name="VFORM">
     <symbol value="INF"/>
    </f>
    <f name="SUBJ">
     <binary value="true"/>
    </f>
   </fs>
   <then/>
   <symbol value="for"/>
  </if>
 </vDefault>
</fDecl>
The AGR feature stores the features relevant to subject-verb agreement. Gazdar et al. specify the range of this feature as CAT. This means that the value is a category, which is their term for a feature structure. This is actually too weak a statement. Not just any feature structure is allowable here; it must be a feature structure for agreement (which is defined in the complete example at the end of the chapter to contain the features of person and number). The following feature declaration encodes this constraint on the value range:
<fDecl name="AGR">
 <fDescr>agreement for person and number</fDescr>
 <vRange>
  <fs type="Agreement"/>
 </vRange>
</fDecl>
That is, the value must be a feature structure of type Agreement. The complete example at the end of this chapter includes the <fsDecl type="Agreement"> which includes <fDecl name="PERS"> and <fDecl name="NUM">.
The PFORM feature indicates the surface form of the preposition used in a construction. Since PFORM is specified above as an open set, string is used in the range specification below rather than symbol.
<fDecl name="PFORM">
 <fDescr>word form of a preposition</fDescr>
 <vRange>
  <vNot>
   <string/>
  </vNot>
 </vRange>
</fDecl>
This example makes use of a negated value: <vNot><string/></vNot> subsumes any string that is not the empty string.

Note that the class model.featureVal includes all possible single feature values, including feature structures, alternations (vAlt) and complex collections (vColl).

⚓18.11.4 Feature Structure Constraints

Ensuring the validity of feature structures may require much more than simply specifying the range of allowed values for each feature. There may be constraints on the co-occurrence of one feature value with the value of another feature in the same feature structure or in an embedded feature structure.

Such constraints on valid feature structures are expressed as a series of conditional and biconditional tests in the fsConstraints part of an fsDecl. A particular feature structure is valid only if it meets all the constraints. The cond element encodes the conventional if-then conditional of boolean logic which succeeds when both the antecedent and consequent are true, or whenever the antecedent is false. The bicond element encodes the biconditional (if and only if) operation of boolean logic. It succeeds only when the corresponding if-then conditionals in both directions are true. In feature structure constraints the antecedent and consequent are expressed as feature structures; they are considered true if they subsume (see section 18.11.3 Feature Declarations) the feature structure in question, but in the case of consequents, this truth is asserted rather than simply tested. That is to say, a conditional is enforced by determining that the antecedent does not (and will never) subsume the given feature structure, or by determining that the antecedent does subsume the given feature structure, and then unifying the consequent with it (the result of which, if successful, will be subsumed by the consequent). In practice, the enforcement of such constraints can result in periods in which the truth of a constraint with respect to a given feature structure is simply not known; in this case, the constraint must be persistently monitored as the feature structure becomes more informative until either its truth value is determined or computation fails for some other reason.

The following elements make up the fsConstraints part of an FSD:

  • fsConstraints (feature-structure constraints) specifies constraints on the content of valid feature structures.
  • cond (conditional feature-structure constraint) defines a conditional feature-structure constraint; the consequent and the antecedent are specified as feature structures or feature-structure collections; the constraint is satisfied if both the antecedent and the consequent subsume a given feature structure, or if the antecedent does not.
  • bicond (bi-conditional feature-structure constraint) defines a biconditional feature-structure constraint; both consequent and antecedent are specified as feature structures or groups of feature structures; the constraint is satisfied if both subsume a given feature structure, or if both do not.
  • then separates the condition from the default in an if, or the antecedent and the consequent in a cond element.
  • iff (if and only if) separates the condition from the consequence in a bicond element.

For an example of feature structure constraints, consider the following ‘feature co-occurrence restrictions’ extracted from the feature system for English proposed by Gazdar, et al. (1985:246–247): 

[FCR 1: [+INV] → [+AUX, FIN]⚓

FCR 7: [BAR 0] ≡ [N] & [V] & [SUBCAT]⚓

FCR 8: [BAR 1] → ~[SUBCAT]]⚓
The first constraint says that if a construction is inverted, it must also have an auxiliary and a finite verb form. That is,
<cond>
 <fs>
  <f name="INV">
   <binary value="true"/>
  </f>
 </fs>
 <then/>
 <fs>
  <f name="AUX">
   <binary value="true"/>
  </f>
  <f name="VFORM">
   <symbol value="FIN"/>
  </f>
 </fs>
</cond>
The second constraint says that if a construction has a BAR value of zero (i.e., it is a sentence), then it must have a value for the features N, V, and SUBCAT. By the same token, because it is a biconditional, if it has values for N, V, and SUBCAT, it must have BAR='0'. That is,
<bicond>
 <fs>
  <f name="BAR">
   <symbol value="0"/>
  </f>
 </fs>
 <iff/>
 <fs>
  <f name="N">
   <binary value="true"/>
  </f>
  <f name="V">
   <binary value="true"/>
  </f>
  <f name="SUBCAT">
   <binary value="true"/>
  </f>
 </fs>
</bicond>
The final constraint says that if a construction has a BAR value of 1 (i.e., it is a phrase), then the SUBCAT feature should be absent (~). This is not biconditional, since there are other instances under which the SUBCAT feature is inappropriate. That is,
<cond>
 <fs>
  <f name="BAR">
   <symbol value="1"/>
  </f>
 </fs>
 <then/>
 <fs>
  <f name="SUBCAT">
   <binary value="false"/>
  </f>
 </fs>
</cond>

Note that cond and bicond use the empty tags then and iff, respectively, to separate the antecedent and consequent. These are primarily for the sake of enhancing human readability.

⚓18.11.5 A Complete Example

To summarize this chapter, the complete FSD for the example that has run through the chapter is reproduced below:
<TEI xmlns="http://www.tei-c.org/ns/1.0">
 <teiHeader>
  <fileDesc>
   <titleStmt>
    <title>A sample FSD based on an extract from Gazdar
         et al.'s GPSG feature system for English</title>
    <respStmt>
     <resp>encoded by</resp>
     <name>Gary F. Simons</name>
    </respStmt>
   </titleStmt>
   <publicationStmt>
    <p>This sample was first encoded by Gary F. Simons (Summer
         Institute of Linguistics, Dallas, TX) on January 28, 1991.
         Revised April 8, 1993 to match the specification of FSDs
         in version P2 of the TEI Guidelines. Revised again December 2004 to
         be consistent with the feature structure representation standard
         jointly developed with ISO TC37/SC4.
    </p>
   </publicationStmt>
   <sourceDesc>
    <p>This sample FSD does not describe a complete feature
         system. It is based on extracts from the feature system
         for English presented in the appendix (pages 245–247) of
         Generalized Phrase Structure Grammar, by Gazdar, Klein,
         Pullum, and Sag (Harvard University Press, 1985).</p>
   </sourceDesc>
  </fileDesc>
 </teiHeader>
 <fsdDecl>
  <fsDecl type="GPSG">
   <fsDescr>Encodes a feature structure for the GPSG analysis
       of English (after Gazdar, Klein, Pullum, and Sag)</fsDescr>
   <fDecl name="INV">
    <fDescr>inverted sentence</fDescr>
    <vRange>
     <vAlt>
      <binary value="true"/>
      <binary value="false"/>
     </vAlt>
    </vRange>
    <vDefault>
     <binary value="false"/>
    </vDefault>
   </fDecl>
   <fDecl name="CONJ">
    <fDescr>surface form of the conjunction</fDescr>
    <vRange>
     <vAlt>
      <symbol value="and"/>
      <symbol value="both"/>
      <symbol value="but"/>
      <symbol value="either"/>
      <symbol value="neither"/>
      <symbol value="nor"/>
      <symbol value="or"/>
      <symbol value="NIL"/>
     </vAlt>
    </vRange>
    <vDefault>
     <binary value="false"/>
    </vDefault>
   </fDecl>
   <fDecl name="COMP">
    <fDescr>surface form of the complementizer</fDescr>
    <vRange>
     <vAlt>
      <symbol value="for"/>
      <symbol value="that"/>
      <symbol value="whether"/>
      <symbol value="if"/>
      <symbol value="NIL"/>
     </vAlt>
    </vRange>
    <vDefault>
     <if>
      <fs>
       <f name="VFORM">
        <symbol value="INF"/>
       </f>
       <f name="SUBJ">
        <binary value="true"/>
       </f>
      </fs>
      <then/>
      <symbol value="for"/>
     </if>
    </vDefault>
   </fDecl>
   <fDecl name="AGR">
    <fDescr>agreement for person and number</fDescr>
    <vRange>
     <fs type="Agreement"/>
    </vRange>
   </fDecl>
   <fDecl name="PFORM">
    <fDescr>word form of a preposition</fDescr>
    <vRange>
     <vNot>
      <string/>
     </vNot>
    </vRange>
   </fDecl>
   <fsConstraints>
    <cond>
     <fs>
      <f name="INV">
       <binary value="true"/>
      </f>
     </fs>
     <then/>
     <fs>
      <f name="AUX">
       <binary value="true"/>
      </f>
      <f name="VFORM">
       <symbol value="FIN"/>
      </f>
     </fs>
    </cond>
    <bicond>
     <fs>
      <f name="BAR">
       <symbol value="0"/>
      </f>
     </fs>
     <iff/>
     <fs>
      <f name="N">
       <binary value="true"/>
      </f>
      <f name="V">
       <binary value="true"/>
      </f>
      <f name="SUBCAT">
       <binary value="true"/>
      </f>
     </fs>
    </bicond>
    <cond>
     <fs>
      <f name="BAR">
       <symbol value="1"/>
      </f>
     </fs>
     <then/>
     <fs>
      <f name="SUBCAT">
       <binary value="false"/>
      </f>
     </fs>
    </cond>
   </fsConstraints>
  </fsDecl>
  <fsDecl type="Agreement">
   <fsDescr>This type of feature structure encodes the features
       for subject-verb agreement in English</fsDescr>
   <fDecl name="PERS">
    <fDescr>person (first, second, or third)</fDescr>
    <vRange>
     <vAlt>
      <symbol value="1"/>
      <symbol value="2"/>
      <symbol value="3"/>
     </vAlt>
    </vRange>
   </fDecl>
   <fDecl name="NUM">
    <fDescr>number (singular or plural)</fDescr>
    <vRange>
     <vAlt>
      <symbol value="sg"/>
      <symbol value="pl"/>
     </vAlt>
    </vRange>
   </fDecl>
  </fsDecl>
 </fsdDecl>
</TEI>

⚓18.12 Formal Definition and Implementation

This elements discussed in this chapter constitute a module of the TEI scheme which is formally defined as follows:

Module iso-fs: Feature structures

The selection and combination of modules to form a TEI schema is described in 1.2 Defining a TEI Schema.

Notes
82
The recommendations of this chapter have been adopted as ISO Standard 24610-1 Language Resource Management — Feature Structures — Part One: Feature Structure Representation
↵
83
See section 9.5.2 Lexical View for more discussion of the need and rationale for ISOcat references.
↵
84
Ways of pointing to components of a TEI document without using an XML identifier are discussed in 16.2.1 Pointing Elsewhere
↵

[English] [Deutsch] [Español] [Italiano] [Français] [日本語] [한국어] [中文]

TEI Consortium | Feedback

TEI Guidelines Version 4.4.0. Last updated on 19th April 2022, revision ff9cc28b0. This page generated on 2022-04-19T20:21:21Z.