friend std::ptrdiff_t operator- (const this_type& i, const this_type& i2) { return i2.distance_to(i); }
friend difference_type operator- (const this_type& i, const this_type& i2) { return i2.distance_to(i); }
BOOST_CONTAINER_FORCEINLINE friend difference_type operator- (const this_type& i, const this_type& i2) { return i2.distance_to(i); }