code: UnitRule<T> getRule(U unit) {
  
          if (unit == null) {
              throw new NullPointerException("Missing chronological unit.");
          }
  
          UnitRule<T> rule = this.unitRules.get(unit);
  
          if (rule == null) {
              if (unit instanceof BasicUnit) {
                  rule = BasicUnit.class.cast(unit).derive(this);
              }
              if (rule == null) {
                  throw new RuleNotFoundException(this, unit);
              }
          }
  
          return rule;
  
      }
docs: <p>Liefert die chronologische Regel zur angegebenen Zeiteinheit. </p>
  
  @param   unit    time unit
  @return  unit rule or {@code null} if not registered