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:
match goal with
           | H:context [ ?P -> False ] |- _ => fold (~ P) in H
           | |- context [ ?P -> False ] => fold (~ P)
           end