Consider complementing the set operations with relational algebra algorithms

For example, natural join:

```
template
OutputIterator set_natural_join(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);

template
OutputIterator set_natural_join(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result,
Compare comp);

template
tuple
set_natural_join_by_key(InputIterator1 keys_first1, InputIterator1 keys_last1, InputIterator2 values_first1,
InputIterator3 keys_first2, InputIterator3 keys_last2, InputIterator4 values_first2,
OutputIterator1 keys_result, OutputIterator2 values_result1, OutputIterator2 values_result2);

template
tuple
set_natural_join_by_key(InputIterator1 keys_first1, InputIterator1 keys_last1, InputIterator2 values_first1,
InputIterator3 keys_first2, InputIterator3 keys_last2, InputIterator4 values_first2,
OutputIterator1 keys_result, OutputIterator2 values_result1, OutputIterator2 values_result2,
Compare comp);
```

With these semantics:

```
template
tuple
set_natural_join_by_key(InputIterator1 keys_first1, InputIterator1 keys_last1, InputIterator2 values_first1,
InputIterator3 keys_first2, InputIterator3 keys_last2, InputIterator4 values_first2,
OutputIterator1 keys_result, OutputIterator2 values_result1, OutputIterator2 values_result2)
{
InputIterator3 keys2_restart = keys_first2;
InputIterator4 values2_restart = values_first2;

for(; keys_first1 != keys_last1; ++keys_first1, ++values_first1)
{
keys_first2 = loop2_restart;
values_first2 = values2_restart;
for(; keys_first2 != keys_last2; ++keys_first2, ++values_first2)
{
if(*keys_first1 < *keys_first2)
{
break;
}
else if(*keys_first2 < *keys_first1)
{
loop2_restart = keys_first;
++loop2_restart;

values2_restart = values2_first;
++values2_restart;
}
else
{
*keys_result = *keys_first;
*values_result1 = *values_first1;
*values_result2 = *values_first2;

++keys_result;
++values_result1;
++values_result2;
}
}
}

return make_tuple(keys_result, values_result1, values_result2);
}
```

Something like `set_natural_join` might make implementing an algorithm like `outer_product` practical.