Goal:
  
  G1 : t
  G2 : t
  on_edge : forall x y : V.t, EdgeOf G1 x y -> EdgeOf G2 x y
  weight_on_edge : forall (x y : V.t) (e : EdgeOf G1 x y),
                   (on_edge x y e).π1 <= e.π1
  x : V.t
  y : V.t
  p : PathOf G1 x y
  ============================
   (weight (map_path p) <= weight p)


DEBUG:Going to execute:
constr:(fun_elim)
Level 5: c
 has value 
(map_path p)