Goal:
  
  G : t
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  p : SPath s0 x y
  t0 : VSet.t
  t1 : V.t
  hin : Disjoint t0 s'
  q : SPath s' t1 z
  ============================
   (VSet.Subset s' (VSet.union t0 s'))


DEBUG:Going to execute:
repeat
            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