Goal:
  
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  x3 : V.t
  a : V.t
  H : VSet.In a _z_
  ============================
   (x3 = a)


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