PROLOG DOUBLY-LINKED LIST PACKAGE
            Contributed by Philip Dart, Melbourne University
                  Received on the 14th of November 1987
                   Shelved on the 6th of December 1987


The entry comes as this file, DLISTS.PRE; the source, DLISTS.PL (after
I've changed it for Edinburgh Prolog); and the original, DLISTS.OLD.


This is my revised version of a doubly-linked list-handling package sent
to the Prolog Digest on 14th of November 1987 by Philip Dart, Melbourne
University. I've changed the syntax slightly so it fits Edinburgh
Prolog: otherwise, the predicates are the same.

Be warned that Philip did send the original package as a joke...
    Following the comments [on the bulletin board] about Fortran as an
    AI language, Melbourne University Department of Artificial
    Intelligence has decided to convert all of its Fortran AI programs
    to NU-Prolog. This package has been written as an aid to this
    conversion.

    Doubly-linked list package: Why use boring old single-linked lists
    when doubly-linked list could make your list processing applications
    run as never before.

    P.S. Don't forget to turn off the occur-check in your version of
    Prolog!

However, it seems to me that the package might be useful in those few
cases where you really _do_ need circular structures (rather than using,
e.g. edge-sets to represent graphs), and Philip has agreed to let me
redistribute his package on this basis. [JNP]

To use the program: consult it, and call the predicates described in the
comments at the top of DLISTS.PL.


Note that the program won't work if you have an occurs-check (very
rare). Also, since these doubly-linked lists are cyclic, they will
probably make 'write' loop indefinitely.


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


PORTABILITY :

    No problems, as long as your Prolog doesn't implement the occurs
    check.               


INTERNAL DOCUMENTATION :

    One-line description of each predicate's specification.