code: @Override
  	protected String initValuesImpl(final FieldCase ca) {
  		double summe = 0;
  		boolean found = false;
  		for (ConstraintWrap constraint : summands) {
  			Double d = constraint.getSum(ca);
  			if (d != null) {
  				found = true;
  				summe += d;
  			}
  		}
  		// a string is only returned if there is at least one value found, otherwise Null
  		if (found) {
  			return format.format(summe);
  		}
  		return null;
  	}
docs: Calls {@link #initValues(FieldCase)} for all summands first. Then the sum of all values of
  every sub-constraint is taken whereupon the numbers are parsed using the format. Finally the
  value is returned formatted as a string using the format object.
  
  @return the sum as string