Proposed resolution:

Change that sentence to use the correct names.

[alg.set.operations] paragraph 1 reads: "The semantics of the set operations are generalized to multisets in a standard way by defining union() to contain the maximum number of occurrences of every element, intersection() to contain the minimum, and so on."

This is wrong. The name of the functions are set_union() and set_intersection(), not union() and intersection().