在scp模拟里看到了这样的代码并大为不解

set <int> s1 = {1, 2, 3}, s2 = {2, 3, 1, 1},
          s3 = {1, 2, 100}, s4 = {3, 4, 5};
map <set <int>, int> mp1, mp2;
...
	cout << (mp1 < mp2) << ' ';
	cout << mp2[s3] << ' ';
	cout << (mp1 <= mp2) << endl;
...

想请问两个map之间的比较逻辑是什么