PROLOG 2-3-TREE PACKAGE
                      Contributed by Mark Johnson
                   Shelved on the 16th of October 1992


The procedures in this file manipulate finite maps from keys to values.
Because 2-3 trees are used, they automatically rebalance after each
insertion or deletion.


Example:

    ?-
    list_to_23( [ cat-dog,
                  hare-rabbit,
                  "hair"-"fur",
                  king-queen,
                  exp(1)-2.71828,
                  3.141593-pi
                ],
                Tree
              ),
    portray_23( Tree ),
    put_23( 39, Tree, steps, Tree1 ),
    portray_23( Tree1 ).

gives

    23 Tree{
      3.141593 - pi
      cat - dog
      hare - rabbit
      king - queen
      exp(1) - 2.71828
      [104, 97, 105, 114] - [102, 117, 114]
    }

    23 Tree{
      3.141593 - pi
      39 - steps
      cat - dog
      hare - rabbit
      king - queen
      exp(1) - 2.71828
      [104, 97, 105, 114] - [102, 117, 114]
    }


There is no routine for deleting items from a tree (you can replace
them), but it would not be hard to write one.


SIZE: 13 kilobytes.

CHECKED ON EDINBURGH-COMPATIBLE (POPLOG) PROLOG : yes.

PORTABILITY : uses % comments; assumes compare/3 for comparing arbitrary
terms. The source file compare_terms.pl in my source for the Logic
Programming Tutor provides this.

INTERNAL DOCUMENTATION : brief specification of public and private
predicates.