let rec gen acc n = match n with
  | 0 -> acc
  | 1 -> Seq.append (seq [1]) acc
  | _ ->
    let n1 = Seq.last acc
    gen (Seq.append acc (seq [(n-1)*n1])) (n-1);;
gen (seq [1]) 5;;

let get_seq = Seq.item (n-1) gen

let fac2_seq = gen (seq [1]) ;;