Exercise 2.69: The following procedure takes as
its argument a list of symbol-frequency pairs (where no symbol appears in more
than one pair) and generates a Huffman encoding tree according to the Huffman
algorithm.


(define (generate-huffman-tree pairs)
  (successive-merge 
   (make-leaf-set pairs)))

Make-leaf-set is the procedure given above that transforms the list of
pairs into an ordered set of leaves.  Successive-merge is the procedure
you must write, using make-code-tree to successively merge the
smallest-weight elements of the set until there is only one element left, which
is the desired Huffman tree.  (This procedure is slightly tricky, but not
really complicated.  If you find yourself designing a complex procedure, then
you are almost certainly doing something wrong.  You can take significant
advantage of the fact that we are using an ordered set representation.)