P5: Guidelines for Electronic Text Encoding and Interchange

TEI: Organization of this Chapter¶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.

TEI: Elementary Feature Structures and the Binary Feature Value¶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 http://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 + | +--- ---+

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.

TEI: Other Atomic Feature Values¶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.

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.

TEI: Feature Libraries and Feature-Value Libraries¶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).

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.

TEI: Feature Structures as Complex Feature Values¶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 a 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.

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

TEI: Re-entrant Feature Structures¶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.

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.

TEI: Collections as Complex Feature Values¶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.

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.

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.

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.

TEI: Feature Value Expressions¶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.

TEI: Default Values¶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.

TEI: Linking Text and Analysis¶18.10 Linking Text and Analysis

TEI: Feature System Declaration¶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:

• It provides a mechanism by which the encoder can list all of the feature names and feature values and give a prose description as to what each represents.
• It provides a mechanism by which the encoder can define constraints not only what it means to be a well-formed feature structure, but also valid feature structure, relative to a given theory stated in typed feature logic. These constraints 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.
• It provides a mechanism by which 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).

[FCR 1: [+INV] → [+AUX, FIN]

FCR 7: [BAR 0] ≡ [N] & [V] & [SUBCAT]

FCR 8: [BAR 1] → ~[SUBCAT]]

TEI: Formal Definition and Implementation¶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

• Elements defined: bicond binary cond default f fDecl fDescr fLib fs fsConstraints fsDecl fsDescr fsdDecl fsdLink fvLib if iff numeric string symbol then vAlt vColl vDefault vLabel vMerge vNot vRange
• Classes defined: model.fsdDeclPart

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