在matlab中输入下面的命令：
help mean2
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 MEAN2 Compute mean of matrix elements.
    B = MEAN2(A) computes the mean of the values in A.
 
    Class Support
    -------------
    A can be numeric or logical. B is a scalar of class double.
 
    Example
    -------
        I = imread('liftingbody.png');
        val = mean2(I)
   
    See also mean, std, std2.

    Reference page in Help browser
       doc mean2
显然matlab中存在这个mean2函数，但是我该怎么确定这个函数属于matlab中的哪个库文件呢 ？？