If either the input .ufm matrix or the internal matrix (assuming it has already been created) are “stacked”, i.e., contain multiple levels of a basic square matrix (see 10.2.4), then various additional operations become possible.
Thus, firstly, a square internal matrix may be created by summing together all the various sub-matrices contained in the stacked .ufm matrix file. (The same operation may also be carried out using an external procedure mxagg; See 10.20.21.)
Secondly, and similarly, a square internal matrix may be created from a single level of the input stacked .ufm file.
Thirdly, if the internal file is a stacked matrix but the nominated input .ufm matrix is square, that matrix may be used to “over-write” or “paste” a particular internal sub-matrix (without any changes being made to any of the other sub-matrices). The paste operation may often be usefully combined with the “cut” operation (see 10.16.6) whereby a square matrix is extracted from a stacked matrix, edited in some fashion and then pasted back into the original matrix.