I still don't understand what's the problem here. User must be aware of the order of a choice and insert his language extension accordingly. Paying some attenti

I still don't understand what's the problem here. User must be aware of the order of a choice and insert his language extension accordingly. Paying some attention to what you're doing never hurts anyway, and it's hardly a blocking issue barring PEG adoption. I normally do this sort of things by providing a number of extension points in the grammar, in the beginning, middle and the end of a semantic entity, instead of simply concatenating grammars (e.g., see such extension points here: https://github.com/combinatorylogic/clike/blob/master/clike/clike-parser.hl https://github.com/combinatorylogic/clike/blob/master/clike/... )