Getting different results than Schrijvers

Anne Ogborn <[email protected]>
Newsgroups gmane.comp.ai.prolog.swi
Message-ID <[email protected]> 
This example by Schrijvers (slide 94 of his presentation online) :- chr_constraint neq/2. neq(X,X) <=> fail.
neq(X,Y) <=> X \= Y | true. ?- neq(a,a).
No
?- neq(a,b).
Yes
?- neq(A,B).
neq(A,B) However in  SWI-Prolog (Multi-threaded, 64 bits, Version 7.1.9-11-g3c17aed) 2 ?- neq(A,B).
false.

Anybody know whats going on?
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