code: public static ErrorMessageFactory shouldBeSubsetOf(Object actual, Object values, Iterable<?> unexpected) {
      return new ShouldBeSubsetOf(actual, values, unexpected);
    }
docs: Creates a new <code>{@link ShouldBeSubsetOf}</code>.
  @param actual the actual set
  @param values the expected superset
  @param unexpected the elements found in the actual set, but not in the given values.
  @return the created {@code ErrorMessageFactory}.