Goal:
    x : V.t  s : VSet.t  y : V.t  H : x = y \/ x <> y
  ============================
   ((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
           | _:~ ?P, H:?P \/ ~ ?P |- _ => clear H
           end