Goal:
    G : t  s0 : VSet.t  s' : VSet.t  x : V.t  t0 : VSet.t
  ============================
   (VSet.Subset s' (VSet.union t0 s'))


DEBUG:Going to execute:
match goal with
           | H:context [ ?x = ?y :> ?T ]
             |- _ =>
                 tryif change T with V.t in H then 
                  fail else
                  tryif change T with VSet.t in H then fail else clear H  
           | H:?P
             |- _ =>
                 tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P)
                  holds by auto  100 with MSet_Prop then 
                  fail else clear H 
           end