Goal:
  
  G : t
  z : V.t
  x : V.t
  y : V.t
  z0 : V.t
  p : PathOf G y z0
  e : EdgeOf G z0 z
  i : VSet.elt
  IHp : VSet.In i (nodes G p) \/ i = z0
  H : VSet.In i (nodes G (PathOf_add_end p e))
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In i (VSet.add x (nodes G p)) \/ i = z0)


DEBUG:Going to execute:
repeat
            match goal with
            | H:?T
              |- _ =>
                  progress change T with V.t in H;
                   repeat
                    match goal with
                    | J:_ |- _ => progress change T with V.t in J
                    | |- _ => progress change T with V.t
                    end
            | H:forall x : ?T, _
              |- _ =>
                  progress change T with V.t in H;
                   repeat
                    match goal with
                    | J:_ |- _ => progress change T with V.t in J
                    | |- _ => progress change T with V.t
                    end
            end