pair pair as key of unordered_map issue

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This happens because there is no specialization for std::tr1::hash<Key> with Key = std::pair<int, int>.
You must to specialize std::tr1::hash<Key> with Key = std::pair<int, int> before declaring tr1::unordered_map<Pair,bool> h;.
This happens because std don’t know how to hash a pair<int, int>.

Following there is a example of how to specialize std::tr1::hash<>

template <>
struct std::tr1::hash<std::pair<int, int> > {
public:
        size_t operator()(std::pair<int, int> x) const throw() {
             size_t h = SOMETHING;//something with x   
             return h;
        }
};

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