Goal:
  
  G : t
  HG : acyclic_no_loop G
  z : V.t
  x : V.t
  e : EdgeOf G x z
  i : VSet.elt
  H : VSet.In i (VSet.add x VSet.empty)
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In i VSet.empty \/ i = x)


DEBUG:Going to execute:
clear H
Level 1: Backtrace:

In environment
G : t
HG : acyclic_no_loop G
z, x : V.t
e : EdgeOf G x z
i : VSet.elt
H : VSet.In i (VSet.add x VSet.empty)
source_vertex0 : VSet.In (s G) (V G)
The term "EdgeOf G x z" has type "Set" while it is expected to have type
 "Prop" (universe inconsistency: Cannot enforce Set <= Prop).