  Let A denote the ring of differential operators on the affine line with its
two usual generators t and d/dt given degrees +1 and -1 respectively. Let X be
the stack having coarse moduli space the affine line Spec k[z] and isotropy
groups Z/2 at each integer point. Then the category of graded A-modules is
equivalent to the category of quasi-coherent sheaves on X.
  Version 2: corrected typos and deleted appendix at referee's suggestion.
