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
  s : VSet.t
  x : V.t
  y : V.t
  p : SPath G1 s x y
  ============================
   (sweight (map_spath p) <= sweight p)


DEBUG:Going to execute:
fun c =>
             let elimc := get_first_elim c in
             let elimfn := match elimc with
                           | fun_elim => constr:(f)
                           end in
             let elimn := match elimc with
                          | fun_elim => constr:(n)
                          end in
             let elimt := make_refine elimn elimc in
             CoreTactics.apply_args c elimt
              ltac:(fun elimc => unshelve refine_ho elimc; cbv beta)