[plt-dev] hangman teachpack broken

The following program triggers the error.

#lang scheme/load

(module draw-sig.ss scheme
   (provide draw^)
   (define-signature draw^()))


(module big-draw.ss scheme

   (require lang/posn
            mzlib/unit
            mred
            mred/mred-unit
            graphics/graphics-sig
            graphics/graphics-posn-less-unit)

   (define-values/invoke-unit/infer
     (export graphics^)
     (link graphics-posn-less@ standard-mred@))

   (provide-signature-elements graphics^)
   )

(module a scheme
   (require 'draw-sig.ss 'big-draw.ss mzlib/unit)
   (provide-signature-elements draw^))

(require 'a)