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 ？？