Filling a matrix with values

I'm a complete beginner to APL, and I had the following question:

Suppose I have a 16x16 matrix of values.  How would I set each value
in the matrix (starting in the second row) equal to the sum of the
matrix element directly above it, and the element to the top-left of
it?  Essentially,

m[x;y]=m[x;y-1]+m[x-1;y-1]

But I'm not sure how to express that in APL.  I've tried entering just
that in APL (with proper symbols, of course), but it doesn't work
because x and y are arrays, and I need to access only two single
elements of the matrix.