PROLOG TURTLE GRAPHICS
        Contributed by Salleh Mustaffa, University of Manchester
                  Received on the 17th of November 1987
                   Shelved on the 6th of December 1987
                  Amended on the 17th Of December 1987


The entry comes as this file, TURTLE.PRE, the source, TURTLE.PL, the
original (before I reformatted it), TURTLE.ORIG, and a _very_ brief
description of my changes, TURTLE.NOTES.


See the notes at the head of TURTLE.PL for documentation. The main
predicate is 'start', which displays some help on the screen, and then
starts the 'turtle command interpreter'.


I've reformatted the program, changed % comments to /* .. */, and made a
few other syntactic changes. I've also changed some calls of 'write' to
calls of 'escape', to avoid embedding escape characters inside strings.


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

    Doesn't use any non-standard predicates, but won't run on our
    (VT100) terminals. However, the appearance of the screen is almost
    correct - small changes to the terminal-handling predicates should
    make it work.


PORTABILITY :

    the cursor-moving commands are terminal-specific.

    Also, if you want to use the 'redo' command, which calls a system
    editor, and then reconsults TURTLE.PL, you'll have to amend the
    'redo' predicate.


INTERNAL DOCUMENTATION :

    The cursor-addressing predicates carry very brief descriptions of
    what they're supposed to do.