A
language described by a set of
transformation rules wherein the
left side of the some or all of the rules indicates that the
transformation of a particular
non-terminal can only take place if the
non-terminal is in a particular relationship to one or more other
symbols. For example, a transformation rule like
(C) A (B) --> foo bar baz
might indicate that A can be transformed into "foo bar baz" but only when it is between C and B.
(The above example doesn't use any well-known convention for describing these rules, 'cuz I don't know a well-known convention anymore.)