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


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