(*s [(ad_simple_path_dist_1 x y l n)] computes the minimum weight of a path 
    from [x] to [y] of maximal length [n] which does not contain vertexes 
    from [l] *)

Fixpoint ad_simple_path_dist_1 (x y : ad) (l : list ad) 
 (n : nat) {struct n} : option D :=
  if Neqb x y
  then Some Dz
  else
   match n with
   | O => None
   | S n' =>
       match MapGet _ cg x with
       | None => None
       | Some edges =>
           let l' := x :: l in (* builds reverse path *)
           all_min
             (fun (z : ad) (d : D) =>
              if ad_in_list z l'
              then None
              else Ddplus (ad_simple_path_dist_1 z y l' n') d) edges
       end
   end.

Lemma ad_simple_path_dist_1_correct_1 : forall (n : nat) (x y : ad) (l : list ad) (d : D), length l <= n -> CG_path y d (x :: l) -> forall prefix : list ad, ad_list_stutters (rev prefix ++ x :: l) = false -> Ddle (ad_simple_path_dist_1 x y prefix n) (Some d) = true.