DOUBLE_QUESTION_MARK

The double-question-mark (⁇) is a single Unicode character (U+2047). It can be placed after a token or after a simple group (a group that has no right side and does not yield anything from its left side). In the case of a token, it allows zero or one occurrences of the token to occur in the Avail source, and discards information about whether the token was encountered. Similarly, when applied to a simple group it accepts zero or one occurrences of the group's left side, discarding the distinction. In both cases, the double-question-mark causes a CompletelyOptional to be built.

This can be useful to make expressions read more fluidly by allowing inessential words to be elided when they would be grammatically awkward.