Match the current element first.
trig and trigger are alternate forms for t.
An _xWILD element or optional element cannot take the trigger element modifier.
Triggering actually causes the rule matcher to match the trigger element first, then move left (i.e., backwards), then move right from the trigger element. This is useful to know in designing rules that combine wildcards with a trigger element, for example.
Note that triggering can be tricky. See the example below, in which, regardless of rule ordering in a pass, an unexpected behavior may occur.
# Attempt to match this rule only when
a _noun node is seen.
_np <- _det _quan _adj _noun [t] @@
# If the following rule is in the same
pass, it will always match first, so that the rule above will never
# be matched! Why? Because the rule matcher will match the rule below whenever an _adj node is found
# (preceded by _det and _quan).
# By the time the _noun node is seen, the rule below has already operated. The presence or absence of [t]
# below makes no difference to this behavior.
_np <- _det _quan _adj [t] @@
optional, lookahead, Phrase Element Modifiers.