Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  a : V.t
  H : x = a -> False
  H0 : a = a
  H2 : x = a
  ============================
   False


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