Goal:
    x : V.t  p : VSet.t  a : V.t  H : VSet.In a p
  ============================
   (x = a)


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