Currently just storing the value without a const. Can do better with
C++11 constructors, so maybe should do that, and cast away const on
compilers without support.
Another problem is that std::allocator<const int> doesn't compile for
libstdc++ (and potentially other standard libraries), so
boost::unordered_set<const int> can't compile. I'm not sure if I should
work around that, as it means changing the type of the container
(i.e. to boost::unordered_set<const int,... , std::allocator<int>>).