27 Algorithms library [algorithms]

27.8 Sorting and related operations [alg.sorting]

27.8.7 Set operations on sorted structures [alg.set.operations] General [alg.set.operations.general]

Subclause [alg.set.operations] 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.