The following features are optional in the transformation language:
The combine-char feature allows character-combination-declarations.
The keyword feature allows #!key in formal-argument-lists.
The multi-source feature allows use of the transform-grove procedure.
The multi-result feature allows multiple result groves.
The regexp feature allows the use of node regular expressions described in “Node Regular Expressions”.
The word feature allows the use of the facilities for word searching described in “Word Searching”.
The hytime feature allows the use of the facilities for HyTime location addressing described in “HyTime Support”.
The charset feature allows the use of the declaration element type forms other than char-repertoire, features, and sgml-grove-plan.