EXCLAMATION_MARK

The exclamation mark (!) can follow an ELLIPSIS (…) to cause creation of a RawTokenArgument, which matches any token that isn't whitespace or a comment.

When it follows an UNDERSCORE (_), it creates an ArgumentForMacroOnly, which matches any A_Phrase, even those that yield ⊤ or ⊥. Since these argument types are forbidden for methods, this construct can only be used in macros, where what is passed as an argument is a phrase yielding that type.

When an exclamation mark follows a guillemet group, that group must contain an alternation. If no alternatives produce a value, the effect is to push a constant corresponding with the alternative's one-based index in the alternation.

If the exclamation mark follows a guillemet group that contains an alternation where at least one alternative produces a value, it attempts to parse a series of alternative phrases. Each parsed phrase is numbered with an ascending one-based subscript, representing the order in which the phrases occurred in the source. The final effect is to push a single tuple with N elements, corresponding to the N alternatives. Each element is itself a tuple with an appropriate number of arguments for that alternative, but preceded by a natural number indicating the order of that phrase in the source.