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When I fix #20303, the new rules will be:
* You can have top-level optional groups anywhere.
* You may nest up to one nested group in a group.
* Whenever you nest a group in another group, all nested groups in that
 stack must favor the same side (left or right).
Here's are example of nesting. This is permitted:
[ a, [b, [c,]]] 
This is not:
[a, [[b,] c,]]
because the nested group adjoining "a" is on the right, but the nested group adjoining "c" is on the left.
The generated names for group variables will probably change to "group_{n}", where n starts at 1 and is assigned going straight across to the right, ignoring nesting, like groups in regular expressions.