Goal:
  
  G : t
  HG : acyclic_no_loop G
  y : V.t
  z0 : V.t
  p : PathOf G y z0
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In z0 (VSet.add z0 (nodes G p)))


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

In environment
G : t
HG : acyclic_no_loop G
y, z0 : V.t
p : PathOf G y z0
source_vertex0 : VSet.In (s G) (V G)
The term "V.t" has type "Type" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce EdgeSet.E.t.u0 <= Prop).