S9 LIB  (makr-rbt procedure)                   ==>  rb-tree
        (rbt-find rb-tree object)              ==>  object | #f
        (rbt-insert rb-tree object1 object2)   ==>  rb-tree
        (rbt-rebuild rb-tree object)           ==>  rb-tree
        (rbt-remove rb-tree object)            ==>  rb-tree

These procedures implement Red-Black Trees.

MAKE-RBT returns an empty tree that uses PROCEDURE as an ordering
predicate. If you plan, for instance, to use strings as keys, use
STRING<? as a predicate.

RBT-FIND locates the value associated with the key OBJECT in the
given RB-TREE and returns it. When the key is not contain in the
tree, it returns #F.

RBT-INSERT returns a new rb-tree with OBJECT2 inserted into Rb-TREE
under the key OBJECT1.

RBT-REBUILD rebuilds the given tree and returns it. The original
tree remains unchanged.

RBT-REMOVE creates a new tree from RB-TREE with the key OBJECT
removed. In fact, this procedure only marks the key as "inactive",
i.e. it is left in the tree, but cannot be found any longer. To
remove inactive nodes, use RBT-REBUILD.

(let ((tree (fold-left
              (lambda (t k)
                 (rbt-insert t k (make-string k #\x)))
              (make-rbt <)
              '(1 2 3 4 5 6 7))))
  (rbt-find tree 5))               ==>  "xxxxx"
