Goal:
    x : VSet.elt  p : VSet.t
  ============================
   (~ VSet.In x p -> VSet.Equal (VSet.remove x (VSet.add x p)) p)


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