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


DEBUG:Going to execute:
progress change T with V.t in H;
            repeat
             match goal with
             | J:_ |- _ => progress change T with V.t in J
             | |- _ => progress change T with V.t
             end