Just a helpful note to others about the CELLS list:
Don't think of these tuples as on a X & Y axis (Cartesian coordinate system). Be sure to think of them as rows and columns. i.e. (4, 4) is like row 4 in column D of a spreadsheet... NOT the coordinate x = 4, y = 4. Don't get confused like I did!
So think of (4, 4) as (column 4, row 4) in a spreadsheet.
Oops, rather (4, 4) is (column 5, row 5) since we start with (0, 0).