Goal:
  
  G : t
  y : V.t
  z0 : V.t
  p : PathOf G y z0
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (True \/ VSet.In z0 (nodes G p))


DEBUG:Going to execute:
match goal with
           | H:context [ ?P -> False ] |- _ => fold (~ P) in H
           | |- context [ ?P -> False ] => fold (~ P)
           end