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    return $self;
}

sub results {
    my $self = shift;
    unless ( defined $self->{_query} ) {
        croak "You can't fetch results because you have not set a query";
    }
    $self->{_engine}->results;
}

sub trace {
    my $self = shift;
    if (@_) {
        $self->{_engine}->trace(shift);
        return $self;
    }
    return $self->{_engine}->trace;
}

sub raw_results {
    my $class = shift;
    if (@_) {
        Engine->raw_results(shift);
        return $class;
    }
    return Engine->raw_results;
}

my $QUOTER;

sub quote {
    my ( $proto, $string ) = @_;
    $QUOTER = Text::Quote->new unless $QUOTER;
    return $QUOTER->quote_simple($string);
}

sub list {
    my $proto = shift;
    return
        join ", " => map { /^$RE{num}{real}$/ ? $_ : $proto->quote($_) } @_;
}

sub continue {
    my $self = shift;
    return 1 unless $self->{_engine};    # we haven't started yet!
    !$self->{_engine}->halt;
}

1;

__END__

=head1 NAME

AI::Prolog - Perl extension for logic programming.

=head1 SYNOPSIS

 use AI::Prolog;
 use Data::Dumper;

 my $database = <<'END_PROLOG';
   append([], X, X).
   append([W|X],Y,[W|Z]) :- append(X,Y,Z).
 END_PROLOG

 my $prolog = AI::Prolog->new($database);
 
 my $list   = $prolog->list(qw/a b c d/);
 $prolog->query("append(X,Y,[$list]).");
 while (my $result = $prolog->results) {
     print Dumper $result;
 }

=head1 ABSTRACT

 AI::Prolog is merely a convenient wrapper for a pure Perl Prolog compiler.
 Regrettably, at the current time, this requires you to know Prolog.  That will
 change in the future.

=head1 EXECUTIVE SUMMARY

In Perl, we traditionally tell the language how to find a solution.  In logic
programming, we describe what a solution would look like and let the language
find it for us.

=head1 QUICKSTART

For those who like to just dive right in, this distribution contains a Prolog
shell called C<aiprolog> and two short adventure games, C<spider.pro> and
C<sleepy.pro>.  If you have installed the C<aiprolog> shell, you can run
either game with the command:

 aiprolog data/spider.pro
 aiprolog data/sleepy.pro

When the C<aiprolog> shell starts, you can type C<start.> to see how to play
the game.  Typing C<halt.> and hitting return twice will allow you to exit.

See the C<bin/> and C<data/> directories in the distribution.

Additionally, you can read L<AI::Prolog::Article> for a better description of
how to use C<AI::Prolog>.  This document is an article originally published in
The Perl Review (L<http://www.theperlreview.com/>) and which they have
graciously allowed me to redistribute.

See also Robert Pratte's perl.com article, "Logic Programming with Perl and
Prolog" (L<http://www.perl.com/pub/a/2005/12/15/perl_prolog.html>) for more
more examples.

=head1 DESCRIPTION

C<AI::Prolog> is a pure Perl predicate logic engine.  In predicate logic,
instead of telling the computer how to do something, you tell the computer what
something is and let it figure out how to do it.  Conceptually this is similar
to regular expressions.

 my @matches = $string =~ /XX(YY?)ZZ/g

If the string contains data that will satisfy the pattern, C<@matches> will