let rec make_index lst =
  match lst with
    [] -> []
  | [(n, kw)] -> List.map (fun s -> (s, [n])) kw
  | (n, kw) :: tl ->
      let map =  List.map (fun (s,n_) ->
      	if List.exists (fun kw_->kw_=s) kw then
      		(s,n::n_)
      	else
      		(s,n_)) (make_index tl)
      in
      (List.filter (fun (kw_,_) ->
      	not (List.exists (fun (s,_) -> s= kw_) map))
      (make_index [n,kw])) @ map
;;

make_index [1,["a";"b"];2,["c";"a"]];;