This section defines all the basic set operations on sorted structures. They also work with multisets ([multiset]) containing multiple copies of equivalent elements. The semantics of the set operations are generalized to multisets in a standard way by defining set_union() to contain the maximum number of occurrences of every element, set_intersection() to contain the minimum, and so on.

```
template<class InputIterator1, class InputIterator2>
bool includes(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2);
template<class InputIterator1, class InputIterator2, class Compare>
bool includes(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
Compare comp);
```

*Returns:*
true
if [first2,last2) is empty or
if every element in the range
[first2,last2)
is contained in the range
[first1,last1).
Returns
false
otherwise.

*Complexity:*
At most
2 * ((last1 - first1) + (last2 - first2)) - 1
comparisons.

```
template<class InputIterator1, class InputIterator2,
class OutputIterator>
OutputIterator
set_union(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
template<class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator
set_union(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);
```

*Effects:*
Constructs a sorted union of the elements from the two ranges;
that is, the set of elements that are present in one or both of the ranges.

*Requires:*
The resulting range shall not overlap with either of the original ranges.

*Returns:*
The end of the constructed range.

*Complexity:*
At most
2 * ((last1 - first1) + (last2 - first2)) - 1
comparisons.

*Remarks:* If [first1,last1) contains m elements that are equivalent to
each other and [first2,last2) contains n elements that are equivalent
to them, then all m elements from the first range shall be copied to the output
range, in order, and then max(n - m, 0) elements from the second range shall
be copied to the output range, in order.

```
template<class InputIterator1, class InputIterator2,
class OutputIterator>
OutputIterator
set_intersection(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
template<class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator
set_intersection(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);
```

*Effects:*
Constructs a sorted intersection of the elements from the two ranges;
that is, the set of elements that are present in both of the ranges.

*Requires:*
The resulting range shall not overlap with either of the original ranges.

*Returns:*
The end of the constructed range.

*Complexity:*
At most
2 * ((last1 - first1) + (last2 - first2)) - 1
comparisons.

*Remarks:* If [first1,last1) contains m elements that are equivalent to
each other and [first2,last2) contains n elements that are equivalent
to them, the first min(m, n) elements shall be copied from the first range
to the output range, in order.

```
template<class InputIterator1, class InputIterator2,
class OutputIterator>
OutputIterator
set_difference(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
template<class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator
set_difference(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);
```

*Effects:*
Copies the elements of the range
[first1,last1)
which are not present in the range
[first2,last2)
to the range beginning at
result.
The elements in the constructed range are sorted.

*Requires:*
The resulting range shall not overlap with either of the original ranges.

*Returns:*
The end of the constructed range.

*Complexity:*
At most
2 * ((last1 - first1) + (last2 - first2)) - 1
comparisons.

*Remarks:*
If
[first1,last1)
contains m
elements that are equivalent to each other and
[first2,last2)
contains n
elements that are equivalent to them, the last
max(m - n, 0)
elements from
[first1,last1)
shall be copied to the output range.

```
template<class InputIterator1, class InputIterator2,
class OutputIterator>
OutputIterator
set_symmetric_difference(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
template<class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator
set_symmetric_difference(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);
```

*Effects:*
Copies the elements of the range
[first1,last1)
that are not present in the range
[first2,last2),
and the elements of the range
[first2,last2)
that are not present in the range
[first1,last1)
to the range beginning at
result.
The elements in the constructed range are sorted.

*Requires:*
The resulting range shall not overlap with either of the original ranges.

*Returns:*
The end of the constructed range.

*Complexity:*
At most
2 * ((last1 - first1) + (last2 - first2)) - 1
comparisons.

*Remarks:*
If [first1,last1) contains m elements that are equivalent to each other and
[first2,last2) contains n elements that are equivalent to them, then
|m - n| of those elements shall be copied to the output range: the last
m - n of these elements from [first1,last1) if m > n, and the last
n - m of these elements from [first2,last2) if m < n.