static Combiner nullSafe(Combiner delegate) {
    Objects.requireNonNull(delegate, "delegate");

    return (left, right) -> {
      if (left == null) {
        return right;
      }

      if (right == null) {
        return left;
      }

      return delegate.combine(left, right);
    };
  }
Combines expression using <a href="https://en.wikipedia.org/wiki/Disjunctive_normal_form">DNF logic</a>
criteria/common/src/org/immutables/criteria/matcher/Combiner.java