[plt-scheme] Teachpacks not working

We got this error when we tried to use world.ss for the animation project:

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Details on the teachpack error.  What I did was click on Language ->
Add Teachpack, then go up 1 folder level to collects, then open
graphics/graphics.ss.  Hit the Run button and and an error pops up:

The teachpack file C:\Program Files\PLT\collects\graphics\graphics.ss
did not load properly.
teachpack: export of posn? from #<path:C:\Program
Files\PLT\collects\graphics\graphics.ss> conflicts with already
existing definitions.

I see that graphics.ss requires graphics-sig.ss, which contains

  (define-signature graphics:posn^
    (make-posn posn? posn-x posn-y))

I'll bet that this is the source of the irritation, but don't know the
cure.

- Mike
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-Andrey