(* Collect the leaves of a binary tree in a list
    A leaf is a node with no successors. Write a predicate leaves/2 to collect them in a list.

    % leaves(T,S) :- S is the list of all leaves of the binary tree T  
*)

type bin_tree = 
		Leaf of string
	|	Node of string * bin_tree * bin_tree
;;

let rec collect_leaves t =
        match t with
                Leaf(s) -> [s]
        |       Node(s,tl,tr) -> List.append (collect_leaves tl) (collect_leaves tr)     
;;