My plan with patmac.arc was to support (o a 5) as one of many sequence parsing utilities inspired by regular expressions. The point is, this particular pattern doesn't describe a value; it describes a subsequence of length 0 or 1.
If you want this to work as part of {a b c} list patterns, I recommend somehow making sure all value patterns can be coerced to (or otherwise interpreted as) length-one subsequence patterns.
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In other news, this kind of (o a 5) syntax means you're putting expressions inside your patterns inside your expressions. You may need to ask yourself just what variables should be in scope in that inner expression; do they include some variables "previously" bound in the pattern? There was no need for this notion of "previously" until now.
