Singleton variable in branch

Hello,
Given the following system session in SWI-Prolog 6.6.1 (Windows, stable
version):

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For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- [p].
Warning: c:/fernan/research/bddeduc/des/des3.3.3/p.dl:1:
        Singleton variable in branch: B
% p.dl compiled 0.00 sec, 2 clauses
true.

2 ?- listing.

:- thread_local thread_message_hook/3.
:- dynamic thread_message_hook/3.
:- volatile thread_message_hook/3.


p(A) :-
        (   functor(A, _, B)
        ->  (   foo(B)
            ->  foo(B)
            ;   foo(B)
            )
        ;   foo(B)
        ).
true.

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I do not understand the warning. I think that B is used in all branches,
isn't it?
In addition, it is said that 2 clauses have been compiled. Is it so? The
source file includes the very same clause as listed.
All the best,
Fernando
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