Introduction to Programming
		   Assignment 7, 10 Nov, 2004
		       Due Fri 19 Nov, 20

                      Balanced search trees

Write a module BalancedSearchTree that implements balanced binary
search trees.  Use the name "BStree a" to denote the datatype.
Your implementation should support the following functions.

a. find :: (BStree a) -> a -> Bool
   -- Reports whether the given value is found in the tree

b. insert :: (BStree a) -> a -> (BStree a)    
   -- Inserts a value into the tree, maintaining logarithmic
   -- height

c. delete :: (BStree a) -> a -> (BStree a)    
   -- Deletes a value from the tree, maintaining logarithmic
   -- height

d. height :: (BStree a) -> Int
   -- Returns the height of the tree

e. size :: (BStree a) -> Init
   -- Returns the number of nodes in the tree

f. isemptytree :: (BStree a) -> Bool
   -- Tells whether the tree is empty

g. emptytree :: (BStree a)
   -- Returns an empty tree

h. buildtree :: [a] -> (BStree a)
   -- Build a search tree in linear time from a sorted list

i. listtree :: (BStree a) -> [a]
   -- List out the values in the tree in ascending order, in
   -- linear time