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:
clear H
Level 1: Backtrace:

In environment
G : t
z, x, y, 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)
The term "EdgeOf G z0 z" has type "Set" while it is expected to have type
 "Prop" (universe inconsistency: Cannot enforce Set <= Prop).