Simplify std::set::erase loop slightly.
Simplify std::set::erase loop slightly.

As of C++11, std::set::erase returns an interator to the subsequent
element, so we don't have to do the a priori post-increment thing.

https://stackoverflow.com/questions/20627458/how-to-remove-elements-from-an-stdset-while-iterating-over-it

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