Breadth-first traversal of a binary tree

type 'a bin_tree =
  Node of 'a bin_tree * 'a * 'a bin_tree | Nil ;;

let bf_iter f = function
    Nil -> ()
  | root ->
      let q = ref [root] in
      while !q <> [] do
        let this = match !q with h :: _ -> h | _ -> Nil
        in let _ = q := match !q with _ :: t -> t | _ -> []
        in match this with
          Node (l, e, r) -> f e; q := !q @ [l; r]
        | _ -> ()
      done ;;