code: public boolean isValid(HistoricDate date) {
  
          if ((date == null) || this.isOutOfRange(date)) {
              return false;
          }
  
          Calculus algorithm = this.getAlgorithm(date);
          return ((algorithm != null) && algorithm.isValid(date));
  
      }
docs: /*[deutsch]
  <p>Ist das angegebene historische Datum g&uuml;ltig? </p>
  
  <p>Wenn das Argument {@code null} ist, liefert die Methode {@code false}. </p>
  
  @param   date    historic calendar date to be checked, maybe {@code null}
  @return  {@code false} if given date is invalid else {@code true}
  @since   3.0