说明：  标示符(IDentifier)是指用来标识某个实体的一个符号。在不同的应用环境下有不同的含义。 在日常生活中,标示符是用来指定某个东西、人,要用到它,他或她的名字;在数学中解方程时,我们也常常用到这样或那样的变量名或函数名
(Identifier is a symbol used to identify an entity. It has different meanings in different application environments. In daily life, the identifier is used to specify a thing or a person, to use it, his or her name; in mathematics, when solving equations, we often use this or that variable name or function name)