Re: setnu-mode and Emacs 21.

[Richard Stallman]

    I thought the cleanest way would be assigning an overlay to each line
    (to just one character in each line, to be precise),

A feature like this should not work by creating hundreds or thousands
of overlays.  It should work through low-level display changes.
If we can display the line number in the mode line, we can put it in
the margin.

That requires C level changes, though.