How's this:

        func : callable
            The transform to be applied. The first parameter of the function
            is the input data. The first two dimensions of the transformed
            data should be (i) vertices and (ii) time.  Transforms which yield
            3D output (e.g. time-frequency transforms) are valid, so long as
            the first two dimensions are vertices and time.  In this case, the
            copy parameter must be True and a list of SourceEstimates, rather
            than a single SourceEstimate, will be returned, one for each index
            of the 3rd dimension of the transformed data.  In the case of
            transforms yielding 2D output (e.g. filtering), the user has the
            option of modifying the input inplace (copy = False) or returning
            a new instance of SourceEstimate (copy = True) with the
            transformed data.