FreeQ
 Usage
• 如果在expr中没有任何表达式匹配form，FreeQ[expr, form]产生True， 否则产生False。
• FreeQ[expr, form, levelspec]仅测试在expr的那些由levelspec指定的层的部分。
Notes
• form 可以是一个模式. • 例如: FreeQ[f[x^2] + y^2, x^_]  . • FreeQ 察看原始表达式的头，测试这些头是否匹配form.
 Further Examples
(同时参见 the Further Examples for the Heads option.) This example shows a feature of FreeQ that is absent from MemberQ: here, FreeQ recognizes that the sum a+c is a subexpression of the larger sum.
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Here we see how the level argument to FreeQ affects its result. When no level is specified, matches at any level are reported.
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