append.ml

let append xs ys =
  let rec helper xs ys acc =
    match xs,ys with
    | [], [] -> List.rev acc
    | [], y::ys' -> helper xs ys' ( y::acc )
    | x::xs', ys -> helper xs' ys ( x::acc ) in
  helper xs ys []


append [1;2;3] [4;5];;
append [] [4;5];;
append [1;2] [];