<fvLib> (feature-value library) assembles a library of reusable feature value elements
(including complete feature structures). 18.4 Feature and Feature-Value Libraries
A feature value library may include any number
of values of any kind, including multiple occurrences of identical
values such as <binary value="true"/> or default. The only
thing guaranteed unique in a feature value library is the set of
labels used to identify the values.