default R isPresent() {
    final UnaryOperator<Expression> expr = e -> Expressions.unaryCall(OptionalOperators.IS_PRESENT, e);
    return Matchers.extract(this).applyAndCreateRoot(expr);
  }
Base interface for optional and optional-derived matchers having just two methods:
{@code isPresent()} and {@code isAbsent()}
criteria/common/src/org/immutables/criteria/matcher/PresentAbsentMatcher.java