There are at least two options here which don't interfere with existing syntax; I'll demonstrate using optional arguments rather than argument conversion for simplicity:
(def foo (x y ('o z)) ...)
This doesn't interfere because binding 'quote is more or less illegitimate in arc. (The reason for this lies in the underlying scheme - if 'quote is lexically bound as an identifier, it doesn't function as a special form.)
(def foo (x y (:o z)) ...)
This doesn't interfere because binding to ':o in arc is more-or-less useless, because ssyntax expands away ':o to 'o. The same is true of '+o.
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.
