Designate the first lookahead element of a rule. The first node matching the lookahead element or to the right of it will be the locus where the rule matcher continues matching.
lookahead is an alternate form for look.
WARNING: A reduce action such as singler or noop should be used to ensure that the lookahead node and nodes to its right are not included in the current rule reduction.
trigger, Phrase Element Modifiers.