#include <cassert>
#include <iostream>
#include <stdexcept>

class Interval {
    int first_;
    int last_;
public:
    Interval(int first, int last) : first_(first), last_(last) {
        if (first > last) throw std::invalid_argument("reversed interval");
    }
    bool contains(int value) const {
        return first_ <= value && value < last_;
    }
};

int main() {
    const Interval work(3, 7);
    assert(work.contains(3));
    assert(!work.contains(7));
    const Interval empty(4, 4);
    assert(!empty.contains(4));
    bool rejected = false;
    try {
        const Interval invalid(7, 3);
        (void)invalid;
    } catch (const std::invalid_argument&) {
        rejected = true;
    }
    assert(rejected);
    std::cout << "interval invariant preserved\n";
}