Count
 Usage
• Count[list, pattern]给出了list中匹配pattern的元素个数。
• Count[expr, pattern, levelspec] 给出了匹配pattern的子表达式的总个数，这些子表达式出现在expr中由levelspec指定的级别上。
Notes
 Further Examples
(See the Further Examples for FullSimplify for a demonstration of how Count can be used to affect the outcome of a simplification.) This counts the number of occurrences of Real numbers in the list. (An Integer is not considered to be Real, any more than a Real is considered to be Complex.)
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This command returns the number of summands that are powers. (Note that the first argument to Count need not be a list.)
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This returns the number of expressions that occur in the sum at any level and are powers. This count is higher; the actual matches can be seen by replacing Count with Cases.
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