REXClassExpressionImpl getREXObjectComplementOf(
			REXClassExpressionImpl complement) {
		// TODO Auto-generated method stub
		if(complement instanceof REXObjectComplementOfImpl)
			return ((REXObjectComplementOfImpl)complement).getClassExpression();
		REXClassExpressionImpl cls = complement.complement;
		if(cls == null)
		{
			cls = new REXObjectComplementOfImpl(complement);
		}
		return cls;
	}