Declare matrices A, B, and C of NXN integers elements and carry out the
	following operations:
	
	i)    C :=  A X B      (store the multiplication of A and B in C)
	ii)    sum := summation of the C elements, column-order.
	--
	
Array2D is rather muddled in its terminology.
It talks about width/height, row/col, and x/y.
The best that can be said for it is that it already exists.
Beware that it is
    array2d at: col at: row
NOT array2d at: row at: col

Does
    (1 to: c width) collect: [:col| (c atCol: col) sum]
do what you want for ii)?  For i), consider the expression
    ((c atRow: i) * (c atCol: j)) sum