Goal:
    s' : VSet.t  t0 : VSet.t  a : V.t  H : VSet.In a s'
  ============================
   (VSet.In a t0 \/ VSet.In a s')


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