code: <V> ElementRule<T, V> getRule(ChronoElement<V> element) {
  
          if (element == null) {
              throw new NullPointerException("Missing chronological element.");
          }
  
          ElementRule<?, ?> rule = this.ruleMap.get(element);
  
          if (rule == null) {
              rule = this.getDerivedRule(element, true);
  
              if (rule == null) {
                  throw new RuleNotFoundException(this, element);
              }
          }
  
          return cast(rule); // type-safe
  
      }
docs: <p>Bestimmt eine chronologische Regel zum angegebenen Element. </p>
  
  @param   <V> Elementwerttyp
  @param   element     chronologisches Element
  @return  Regelobjekt
  @throws  RuleNotFoundException if given element is not registered in
  this chronology and there is also no element rule which can
  be derived from element