#include <iostream>
#include <set>
 
struct Foo
{
    int _x, _y, _z;
    Foo(int x, int y, int z): _x(x), _y(y), _z(z) {}
    bool operator<(const Foo& rhs) const
    {
        return (_x < rhs._x) && (_x + _y < rhs._x + rhs._y) &&
               (_x + _y + _z < rhs._x + rhs._y + rhs._z) ; // majorization
    }
};
 
int main()
{
    std::set<Foo> setFoo;
    // try to insert 2 equivalent elements
    setFoo.insert(Foo{1, 2, 3}); 
    if(setFoo.insert(Foo{1, 2, 0}).second == false) // failed to insert
        std::cout << "Equivalent element already in the set!" << std::endl;
}
 

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