Fix comment typo #comment #typo
SupSub blocks deleted one at a time inst. of unwrapping

See new test case for example.

So that no less convenient than before, when SupSub's were separate
commands respace'd on top of each other.

Unfortunately, a SupSub instance can now go from being subscript-only to
being superscript-only, so .addBlock() has to dynamically check what
kind the current instance is, rather than using polymorphism.