how does one work this out if there is a zero in one or in both numbers... I played around with this on a napkin at a bar and couldn't figure it out...
Imagine (or draw) a dotted line where the zeroes are..Count the dotted intersections as contributing zero to the diagonal addition step.
I guess you'd just have to figure out some way to represent it, say a squiggly line or something, then remember not to count it's intersection, or you could just leave a big blank spot, but too many big blanks might throw off the ease of vertical grouping