docs: public void assertExactlyOneDefined(final String param1, final String param2) {
      // Asserting that exactly one is defined is the same as asserting that they do not have the same
      // value for definedness.
      if (isPresent(param1) == isPresent(param2)) {
        throw new ParameterException(
            String.format("Exactly one of %s and %s must be defined.", param1, param2));
      }
    }