gprolog display incorrectly a ?

Michel Levy <[email protected]>
Newsgroups gmane.comp.gnu.prolog.general
Message-ID <[email protected]>
Let's suppose that the predicate a is defined by
a(f(b),1).
a(f(c),2).
We execute the query
?- a(f(b),X).
X = 1?
Why Prolog display a ? although there are no remaining alternatives ?

-- 
Michel Levy
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email : [email protected]
http://www-lsr.imag.fr/users/Michel.Levy/
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