I think I disagree. What he wanted can certainly be called a matrix. An m-by-n matrix has an entry for every (x, y), 0<=x<m 0<=y<n, but that's not a description

I think I disagree. What he wanted can certainly be called a matrix. An m-by-n matrix has an entry for every (x, y), 0<=x<m 0<=y<n, but that's not a description of the way to represent a matrix in memory. That is an implementation decision. Of course I wasn't there, and I don't know the fine details that got you mad.