docs: private static boolean isDowntoEligible(Temporal from, Temporal to) {
          TemporalAmount amount = rightShift(from, to);
          if (amount instanceof Period) {
              return isNonpositive((Period) amount);
          } else if (amount instanceof Duration) {
              return isNonpositive((Duration) amount);
          } else {
              throw new GroovyRuntimeException("Temporal implementations of "
                      + from.getClass().getCanonicalName() + " are not supported by downto().");
          }
      }