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Задача 1 (Exercise P11.4).
Modify the merge sort algorithm to sort a vector of employees by salary.


Задача 2 (Exercise P13.2).
Write functions

set<int> set_union(set<int> a, set<int> b)
set<int> intersection(set<int> a, set<int> b)

that compute the set union and intersection of the sets a and b.
(Don’t name the first function union—that is a reserved word in C++.)