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:
match goal with
           | H:VSet.In ?x ?r -> False
             |- VSet.In ?x ?s -> False =>
                 contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
           | H:VSet.In ?x ?r -> False
             |- ?x = ?y -> False =>
                 contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
           | H:VSet.In ?x ?r -> False
             |- ?y = ?x -> False =>
                 contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
           | H:?P -> False
             |- ?Q -> False =>
                 tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_elt_Prop P)
                  holds by auto  100 with MSet_Prop then
                  (contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body)
                   else VSetDecide.MSetDecideAuxiliary.fsetdec_body 
           | |- _ => VSetDecide.MSetDecideAuxiliary.fsetdec_body
           end