I wrote the xprosper.sty package from the documentclass prosper.cls
belonging to the prosper distribution:

        http://prosper.sourceforge.net/

(A copy has been inserted here for comparison). 

The prosper package should be used with the seminar documentclass:

 * Unlike the prosper class, it does not redefine the background, 
   or the page setting.

 * As the prosper class, it enables a special form of overlays.

 * It fixes a few problems, mostly blank characters, from the original
  package.

 * Overlays can be refered to by relative positions (in addition to absolute
   positions), such that moving blocks of text in a slide does not break the
   overlays.

 * No need to count the number of overlays yourself, it does it for you.

Didier Remy