Creating Balanced Trees from Sorted Lists
Find the middle of the list:
         (1 2 3 4 5 6 7 8 9 10)
Use BST constructors to build a BST with:
-  6 as its root
     
 
-  a balanced BST built out of (1 2 3 4 5)
     as its left subtree
     
 
-  a balanced BST built out of (7 8 9 10)
     as its right subtree
This is Exercise 8.12, p. 224.  Helpful procedures:
-  list-ref: built-in
     
 
-  first-elements-of: defined in ch. 7
     
 
-  list-tail: built-in