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MAT #MERGE A,B TO C
combines two m x n matrices A and B to an 2m x n matrix C
by merging rows. Thus the rows of C will be:
row 1:    row 1 of A
row 2:    row 1 of B
row 3:    row 2 of A
row 4:    row 2 of B
...       ...
row 2m-1: row m of A
row 2m:   row m of B

MAT #MERGE A,B TO C BY mA,mB
combines an mA*k x n matrix A and an mB*k x n matrix B
to an (mA+mB)*k x n matrix C by merging rows of A and B
in blocks of rows of mA and mB, respectively.
Thus the rows of C will be:
row 1:       row 1    of A
row 2:       row 2    of A
...          ...
row mA:      row mA   of A
row mA+1:    row 1    of B
row mA+2:    row 2    of B
...          ...
row mA+mB:   row mB   of B
row mA+mB+1: row mA+1 of A
row mA+mB+2: row mA+2 of A
...

A = More about additional MAT #operations 
M = More about MAT operations