I didn't find it yet, but I'd be very surprised if you couldn't just define a new "_is_" method/function/block/parse extension/whatever the hell it is to mean "

I didn't find it yet, but I'd be very surprised if you couldn't just define a new "_is_" method/function/block/parse extension/whatever the hell it is to mean "::=". Or maybe "const_is_" if "_is_" wouldn't work. Anyway, Avail looks very, very interesting, like it's a combination of Smalltalk, REBOL and Coq (although it's just an uneducated impression for now).