let mulVectors v1 v2 = List.fold2(fun s x y -> s + x * y) 0 v1 v2
let mulMatr m1 m2 =
    let rec mulMatr'' m1 m2 m3 =
        let rec mulMatr' m1 m2 v =
            match m1, m2 with
            | h1::t1, h2::t2 -> mulMatr' t1 m2 ((mulVectors h1 h2)::v)
            | [], _ -> List.rev v
            | _ -> failwith "error in format"
        match m1, m2 with
        | h1::t1, h2::t2 -> let v = mulMatr' m1 m2 []
                            mulMatr'' m1 t2 (v::m3)
        | _, [] -> List.rev m3
        | _ -> failwith "error in format"
    mulMatr'' m1 m2 []