Goal:
    x : V.t  s : VSet.t  y : V.t
  ============================
   ((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:
match goal with
           | J:_ |- _ => progress change T with V.t in J
           | |- _ => progress change T with V.t
           end