11.1 Features
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 section 10.3.2, Node Regular Expressions.

•  The word feature allows the use of the facilities for word searching described in section 10.3.1, Word Searching.

•  The hytime feature allows the use of the facilities for HyTime location addressing described in section 10.2.1, HyTime Support.

•  The charset feature allows the use of the declaration element type forms other than char-repertoire, features, and  sgml-grove-plan.