What does an apostrophe do?

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Article #287

#287

From: Martin Sandman
Date: Sat, 19 Aug 1989 15:51

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A Prolog SEND MORE MONEY program was posted here sometime ago.
I am trying to port it to UNSW Prolog.  I don't know what flavor
of Prolog the original is but an apostrophe is used as follows:
     plus(x',y,z'):-
       plus-one(x,x'),
       plus(x,y,z),
       plus-one(z,z').
What does the apostrophe mean?  Thanks in advance.  -- Martin
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     ______________________________________________________________________
     ______________________________________________________________________
     = UUCP:   sdcsvax!ncr-sd!fiddler!martins      Martin D. Sandman      =
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Path: arc1-ca.pugleaf.net!utzoo!attcan!uunet!cs.utexas.edu!usc!ucsd!nosc!chip!ncr-sd!booboo!martins
From: martins@booboo.SanDiego.NCR.COM (Martin Sandman)
Newsgroups: comp.lang.prolog
Subject: What does an apostrophe do?
Message-ID: <1652@ncr-sd.SanDiego.NCR.COM>
Date: 19 Aug 89 15:51:54 GMT
Sender: news@ncr-sd.SanDiego.NCR.COM
Reply-To: martins@booboo.SanDiego.NCR.COM (Martin Sandman)
Distribution: comp.lang.prolog
Organization: NCR Corporation, Rancho Bernardo
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