let rec appendtolist l a =
  match l with
  |[] -> [a]
  |h :: t -> (h :: appendtolist t a)
;;

let insert l a b =
  if List.mem_assoc a l
  then
    let n = List.assoc a l in (a, (appendtolist n b))::(List.remove_assoc a l)
  else (a, (appendtolist [] b))::l
;;