I personally like the :o, maybe just because it reminds me of CL. Anyway, it shouldn't be too hard to add a special case for ': in ssyntax if it is the first character.
After all, wouldn't that be an error anyway? Therefore redefining it would not interfere with normal usage of ':. In fact, it makes sense to have different versions for each ssyntax if it is at the beginning, end or middle.
