public Set<Integer> voterIds() {
        return voters.keySet();
    }
@Test
    void testVoterIds() {
        VoterSet voterSet = VoterSet.fromMap(voterMap(IntStream.of(1, 2, 3), true));
        assertEquals(new HashSet<>(Arrays.asList(1, 2, 3)), voterSet.voterIds());
    }