[comp.lang.prolog] What does an apostrophe do?

martins@booboo.SanDiego.NCR.COM (Martin Sandman) (08/19/89)

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      =
     = NCR:    Martin.Sandman@SanDiego.NCR.COM     NCR Corporation        =
     = Phone:  (619) 485-3554                      16550 West Bernardo Dr.=