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


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