code: static public BooleanExpression NOT(BooleanExpression operand){
  		BooleanExpression result = new BooleanExpression(OPERATOR_NOT);
  		result.addOperand(operand);
  		return result;
  	}
docs: 创建用NOT修饰的表达式
  @param operand
  @return	用NOT修饰之后的表达式