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 : V.t
  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:forall a : V.t, _, _:context [ VSet.In ?x _ ]
              |- _ => let P := type of (H x) in
                      assert
                      new
                      P
                      by
                      exact
                      (H x)
            | H:forall a : V.t, _
              |- context [ VSet.In ?x _ ] =>
                  let P := type of (H x) in
                  assert
                  new
                  P
                  by
                  exact
                  (H x)
            | H:forall a : V.t, _, _:context [ ?x = _ ]
              |- _ => let P := type of (H x) in
                      assert
                      new
                      P
                      by
                      exact
                      (H x)
            | H:forall a : V.t, _
              |- context [ ?x = _ ] =>
                  let P := type of (H x) in
                  assert
                  new
                  P
                  by
                  exact
                  (H x)
            | H:forall a : V.t, _, _:context [ _ = ?x ]
              |- _ => let P := type of (H x) in
                      assert
                      new
                      P
                      by
                      exact
                      (H x)
            | H:forall a : V.t, _
              |- context [ _ = ?x ] =>
                  let P := type of (H x) in
                  assert
                  new
                  P
                  by
                  exact
                  (H x)
            end