ClearAll[symb1, symb2, ...]清除与符号相关联的所有的值、定义、属性、信息或缺省值.
ClearAll["form1", "form2", ...]清除所有名称与任何一个 匹配的符号.
定义符号的值和属性：
清除值和属性：
Clear ClearAttributes Remove
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