Re: capturing print_message to a file

Thanks for the example below.  I was aware of these methods, but 
after reading documentation I didn't understand how to use them.

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In fact, you can also redirect user_error.  Boils down to this:

current_error(Stream) :-
    stream_property(Stream, alias(user_error)), !. % force det.

set_error(Stream) :-
    set_stream(Stream, alias(user_error)).

Now, you can do this:

:- meta_predicate
    redirect_error(0, +).

redirect_error(Goal, File) :-
    current_error(OldErr),
    setup_call_cleanup(
        open(File, write, Err),
        setup_call_cleanup(
        set_error(Err),
        once(Goal),
        set_error(OldErr)),
        close(Err)).
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