namespace detail 
{
    template< InputIterator I1, SizedSentinel<I1> S1,
              InputIterator I2, SizedSentinel<I1> S2 >
    bool check_size(I1& first1, S1& last1, I2& first2, S2& last2)
    {
        return last1 - first1 != last2 - first2;
    }
 
    template< InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I1> S2 >
    bool check_size(I1& first1, S1& last1, I2& first2, S2& last2)
    {
        return false;
    }
}
 
template< InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I2> S2,
          class Pred = ranges::equal_to<>, 
          class Proj1 = ranges::identity, class Proj2 = ranges::identity >
  requires IndirectlyComparable<I1, I2, Pred, Proj1, Proj2>
bool equal(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = Pred{},
           Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{}) 
{
    if(detail::check_size(first1, last1, first2, last2)) return false;
    for(; first1 != last1 && first2 != last2; (void) ++first1, (void)++first2) {
        if(!ranges::invoke(pred, ranges::invoke(proj1, *first1), 
                                 ranges::invoke(proj2, *first2))) {
            return false;
        }
    }
    return first1 == last1 && first2 == last2;
}