Added comment

libraries/chain/include/graphene/chain/account_object.hpp

@@ -278,6 +278,17 @@ namespace graphene { namespace chain {
     */
    class account_member_index : public secondary_index
    {
+      /* std::less::operator() is less efficient so using key_compare here.
+       * Let assume that it has two keys key1 and key2 and we want to insert key3.
+       * the insert function needs to first call std::less::operator() with key1 and key3.
+       * Assume std::less::operator()(key1, key3) returns false.
+       * It has to call std::less::operator() again with the keys switched, 
+       * std::less::operator()(key3, key1), to decide whether key1 is equal to key3 or 
+       * key3 is greater than key1. There are two calls to std::less::operator() to make
+       * a decision if the first call returns false.
+       * std::map::insert and std::set used key_compare, 
+       * there would be sufficient information to make the right decision using just one call.
+       */
       class key_compare {
       public:
          inline bool operator()( const public_key_type& a, const public_key_type& b )const