Re: something like linum.el ought to be added

[Richard Stallman]

    > If each overlay records which windows it is meant for, the
    > window-scroll-functions can could delete and redo only those that
    > pertain to the current window.  Each window will run the
    > window-scroll-functions to recompute the overlays it needs.

    And the ?window? overlay property can allow the overlays for the
    different windows to be completely independent of each other.  There
    could be two overlays for a given line and only one would be displayed
    in each window.

It could be used for that, in another feature where that was needed.
However, each line's number is the same in all windows, so linum
doesn't need to do this.

    > (We can add a primitive to select overlays based on a certain
    > property value, to make that faster.)

    Like next-single-property-change except for overlays instead of text
    properties?

The same basic idea, yes, but a different interface might be more
suitable here.

    But maybe Richard was rather thinking about a more discriminative form
    of overlays-at.

Rather, a more discriminative form of overlays-in.