Added trailing sep as unresolved question

Author: mark-i-m

text/0000-macro-at-most-once-rep.md

@@ -170,4 +170,5 @@ Finally, we could do nothing and wait for macros 2.0. However, it will be a whil
 
 Unresolved Questions
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-None that I can think of...
+
+- Should the `?` Kleene operator accept a separator? Adding a separator is completely meaningless (since we don't accept trailing separators, and `?` can accept "at most one" repetition), but allowing it is consistent with `+` and `*`. Currently, we allow a separator. We could also make it an error or lint.