#include <algorithm>
#include <cassert>
#include <set>

struct ByBucket {
    bool operator()(int left, int right) const noexcept {
        return left / 10 < right / 10;
    }
};

int main() {
    std::set<int, ByBucket> representatives{12, 31};
    const auto equivalent = representatives.find(18);
    assert(equivalent != representatives.end());
    assert(*equivalent == 12);
    assert(std::find(representatives.begin(), representatives.end(), 18)
           == representatives.end());
    const auto inserted = representatives.insert(19);
    assert(!inserted.second);
    assert(representatives.size() == 2);
    assert(!ByBucket{}(12, 12));
    assert(!ByBucket{}(12, 18));
    assert(!ByBucket{}(18, 12));
}