OCaml Binary Tree Catamorphism

type 'a tree = Br of 'a * 'a tree * 'a tree | Lf

let rec cata map_lf map_br t =
    match t with
    | Lf -> map_lf ()
    | Br (x, l, r) ->
        let reduced = cata map_lf map_br (Br (x, l, r)) in
        map_br reduced

let fold_right op tree init = cata
    (fun unit -> init)
    (fun value -> op value)
    tree

let size t = fold_right (fun x -> 1 + x) t 0