Looks like increment will add another character upon overflow, presumably duplicating the kind of the previous leftmost. To undo ++, the -- could eat the leftmost upon underflow. But ++ can't always be the exact opposite since it doesn't know which kind of character to add.

I'm aware that I'm clamping on underflow. It's a snippet, not a polished module! It was done more for the challange, but if I really needed this (like the poster that inspired it), I'd have the function croak or something, if this was significant to the program.

—John