Goal:
  
  x : V.t
  s : VSet.t
  y : V.t
  H0 : x = y -> False
  H : VSet.In y s
  H2 : x = y -> False
  ============================
   (x = y)


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