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


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