text: #include <iostream>
  #include <algorithm>
  #include <unordered_map>
  #include <unordered_set>
  #include <functional>
  
  
  int main()
  {
  	//**** Note ****
  	//**** Important. Although in standard we cannot insert pair to unordered_set, we can do so for std::multiset. See example in find K pairs of smallest sum. 
  	std::unordered_set<std::pair<int, int>> hello;
  	//There is no standard way to insert pair to unordered_set.
  	//Check "insert pair into unordered_set" for the ways of providing specialize the template to do so. 
  	//**** Seems not necessary, because this is what exactly does by un_ordered_map.
  	return 0;
  }
  