Goal:
    x : V.t  s : VSet.t  y : VSet.elt
  ============================
   ((VSet.In y (VSet.add x s) -> x = y \/ VSet.In y (VSet.remove x s)) /\
    (x = y \/ VSet.In y (VSet.remove x s) -> VSet.In y (VSet.add x s)))


DEBUG:Going to execute:
progress change T with V.t in H;
            repeat
             match goal with
             | J:_ |- _ => progress change T with V.t in J
             | |- _ => progress change T with V.t
             end