code: private void appendExpression(final BooleanExpression newExpression) {
  		if (lhsExpression != null) {
  			throw new SyntaxException("Syntax exception: use AND or OR to join expressions");
  		}
  
  		lhsExpression = newExpression;
  
  		GroupExpression currentExpression = stack.isEmpty() ? null : stack.peek();
  
  		if (currentExpression == null) {
  			this.expression.setExpression(newExpression);
  		} else {
  			currentExpression.add(newExpression);
  		}
  	}
docs: Appends the given expression to this expression.
  
  @param newExpression the expression to append.