Thinking about the paired-o suggestion I made, it has an advantage over grouping single optional arguments by their own parens -- at least if you have more than 2 optional parameters :). Compare:
(a b (opt-c default-c))
(a b (o opt-c default-c))
(a b (opt-c default-c) (opt-d default-d))
(a b (o opt-c default-c opt-d default-d))
(a b (opt-c default-c) (opt-d default-d) (opt-e default-e))
(a b (o opt-c default-c opt-d default-d opt-e default-e))
It's like a 3-character &optional, just with one of the characters at the end. However, you'd need to specify nil defaults by hand (though you might let an uneven pair at the end default to nil). E.g.,
(def markdown (s (o maxurl) (o nolinks))
...)
could, at best, become
(def markdown (s (o maxurl nil nolinks))
...)
But paired-o also generalizes the current behavior, so we could still use the original signature.
