Goal:
  
  G : t
  y : V.t
  z0 : V.t
  p : PathOf G y z0
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In z0 (VSet.add z0 (nodes G p)))


DEBUG:Going to execute:
tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P) holds by
            auto  100 with MSet_Prop then fail else clear H