Accompanying material for the paper "Generalizing Generalized Tries"
by Ralf Hinze, in Journal of Functional Programming, 10(4),
pp. 327-351, July 2000.

External binary search trees.

> module Bintree (
>     Bintree(Leaf, Node),
> ) where

> data Bintree a1 a2		=  Leaf a1
>				|  Node (Bintree a1 a2) a2 (Bintree a1 a2)
>				   deriving (Show)