code: private int getNextConstraintIndex() throws PropertyException{
  		Set<Integer> usedIndexes = getConstraintNameIndexes();
  		int nextIndex = 1;
  		while(usedIndexes.contains(nextIndex)){
  			nextIndex++;
  		}
  		return nextIndex;
  	}
docs: Returns the lowest unused index for constraint names.<br>
  Constraint names are enumerated (CONSTRAINT_1, CONSTRAINT_2, ...).<br>
  When new constraints are added, the lowest unused index is used as property name.
  @return The lowest free index to be used for constraint naming.
  @throws PropertyException if the extraction of used indexes fails.