<fvLib> (biblioteca de valores de rasgo) reúne una biblioteca de elementos de valor de rasgo
reutilizables (incluyendo estructuras de rasgo completas). [19.4 Feature Libraries 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.