type 'a btree = 
  | Leaf of 'a
  | Branch of ('a btree) * ('a btree)


let rec flatten = function
  | Leaf(a) -> [a]
  | Branch(r, l) -> flatten r @ flatten l


let _ =
  Branch(Leaf "a", Branch(Leaf "b", Leaf "c"))   
  |> flatten
  |> List.iter (Printf.printf "%s ")
  ;