A public rule may also be imported into other grammars. All non-public rules are private. Private rules can be referenced only by other rules within the same grammar but they can reference public rules imported from other grammars. Most importantly a rule defines an expansion that declares how the rule is expanded into words, references to other rules and patterns of words and references. Rules and Tokens Words, or more precisely tokens , are the basic units of a grammar and indicate those things that a user can say.
Any token is a legal expansion in a rule definition.
Sequences of individual tokens are separated by white space and the sequence is a legal expansion. A rule reference is equivalent to a non-terminal reference in a traditional grammar. The referenced rule may be any public rule of another grammar in which case a relative URI or absolute URI is used. A sequence of legal expansions is itself a legal expansion. As mentioned previously, tokens in sequence should be separated by white space.