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:
match constr:(n) with
           | 0%nat => uconstr:(c)
           | Datatypes.S ?n => make_refine n ltac:(uconstr:((c _)))
           end