Vertical margin bars

[Ludovic Courtès]

Hi,

Has anyone ever thought about implementing vertical margin bars?  For
instance, the following example:

  This is introductory text.

  @StartMarginBar
  This is text with a vertical margin bar alongside.
  @EndMarginBar

  This text is not along the margin bar.

would yield something like:

     This is introductory
   | text.  This is text with
   | a vertical margin bar
   | alongside.  This text is
     not along the margin
     bar.

This is very useful to represent differences between two versions of a
document, for instance.

I wonder how this could be implemented, though.  Margin footnotes could
probably serve as a starting point.  But then the vertical lines need to
automatically span over several columns/pages as needed (like table
borders do).  Ideas?

Thanks,
Ludovic.