The LISP parser my co-worker built had the ability to encode in a grammar rule which "corner" to trigger that rule on. Often it was the head of the phrase, but

The LISP parser my co-worker built had the ability to encode in a grammar rule which "corner" to trigger that rule on. Often it was the head of the phrase, but it could be things like the word "and" (no need to trigger a conjunction rule unless there's a conjunction).