DeleteCases
 Usage
•DeleteCases[expr, pattern] 用来删除 expr中与 pattern匹配的所有元素.
• DeleteCases[expr, pattern, levspec] 用来删除expr的 levspec 层上与 pattern匹配的所有部分.
Notes
• 利用可选项 Heads -> True，你能使用DeleteCases来删除头部. 删除一个表达式中特定元素的头等价于在那个位置把FlattenAt应用到该表达式.
 Further Examples
This deletes all elements of the list that are real numbers.
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This command deletes all integers from levels 1 to 2.
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In contrast this command deletes all integers only at level 2.
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