Goal:
  
  G : t
  HG : acyclic_no_loop G
  z : V.t
  x : V.t
  y : V.t
  z0 : V.t
  e0 : EdgeOf G x y
  p : PathOf G y z0
  e : EdgeOf G z0 z
  IHp : ~~ VSet.mem z0 (nodes G p) -> is_simple G (PathOf_add_end p e)
  nmen : is_true (~~ VSet.mem x (nodes G p))
  iss : is_true (is_simple G p)
  Hm : ~ VSet.mem z0 (VSet.add x (nodes G p))
  nmem : VSet.In z0 (nodes G p)
  edges_vertices0 : forall e : EdgeSet.elt,
                    EdgeSet.In e (E G) ->
                    VSet.In e..s (V G) /\ VSet.In e..t (V G)
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.E.eq z0 x \/ VSet.In z0 (nodes G p))


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