Goal:
    _z_ : VSet.t  a : V.t
  ============================
   ((VSet.In a _z_ -> VSet.In a _z_) /\ (VSet.In a _z_ -> VSet.In a _z_))


DEBUG:Going to execute:
repeat
            match goal with
            | H:?x = ?x |- _ => clear H
            | H:?x = ?y |- _ => rewrite -> H in *; clear H
            end