@Test
  void absentPresent() {
    String present = "{$and: [{optional: {$exists: true}}, {optional: {$ne: null}}]}";
    String absent = "{$or: [{optional: {$exists: false}}, {optional: null}]}";
    check(criteria.optional.isPresent()).matches(present);
    check(criteria.optional.isAbsent()).matches(absent);

    // negation of absent / present
    check(criteria.optional.not(PresentAbsentMatcher::isAbsent)).matches(present);
    check(criteria.optional.not(PresentAbsentMatcher::isPresent)).matches(absent);
  }
{@code in} / {@code notIn} with single element converted to {@code eq} / {@code ne}
criteria/mongo/test/org/immutables/criteria/mongo/FindVisitorTest.java