Perform an optional match, executing Default if Match is not
matched. This is comparable to ignore/1. Both Match and Default are
DCG body terms. Default is typically used to instantiate the output
variables of Match, but may also be used to match a default
representation. Using [] for Default succeeds without any
additional actions if Match fails. For example:
?- phrase(optional(number(X), {X=0}), `23`, Tail).
X = 23,
Tail = [].
?- phrase(optional(number(X), {X=0}), `aap`, Tail).
X = 0,
Tail = `aap`.
