"Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines."

因为返回 bool 要保证 comp(a, b) 和 comp(b, a) 是只有一个 true 的，在相等时候不可以返回 true