default R contains(V value) {
    return Matchers.extract(this).applyAndCreateRoot(e -> Expressions.binaryCall(IterableOperators.CONTAINS, e, Expressions.constant(value)));
  }
Matcher on {@link Iterable} types. Has methods like {@code isEmpty()} / {@code notEmpty()}
and others.
criteria/common/src/org/immutables/criteria/matcher/IterableMatcher.java