Match (MatchY on success, MatchN on failure). If MatchY is returned it has "matches" property with values is suitable order to call c. If MatchN is returned, it has "message" property explaining why there was no match. Currently it's not printed anywhere.
Example
ArgvMatcher; (["--b", "B", "A", "C"] ~ F(a,b,c) 7).matches # %[A B C]
(["A", "C", "D", "--b", "B"] ~ ArgvMatcher("positionals", "a") do F(a,b) 7).matches # %[%[A C D] B]