Advanced feature - provide direct access to a subset of matcher based a set of known encoding lengths. Lengths are provided in code units. The sub-matcher then may do less operations per any test/match.

Use with care as the sub-matcher won't match any $(CODEPOINTS) that have encoded length that doesn't belong to the selected set of lengths. Also the sub-matcher object references the parent matcher and must not be used past the liftetime of the latter.

Another caveat of using sub-matcher is that skip is not available preciesly because sub-matcher doesn't detect all lengths.