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package AI::Prolog::Engine;
$REVISION = '$Id: Engine.pm,v 1.13 2005/08/06 23:28:40 ovid Exp $';
$VERSION  = '0.4';
use strict;
use warnings;
use Carp qw( confess carp );

use Scalar::Util qw/looks_like_number/;
use Hash::Util 'lock_keys';

use aliased 'AI::Prolog::Term';
use aliased 'AI::Prolog::Term::Cut';
use aliased 'AI::Prolog::Term::Number';
use aliased 'AI::Prolog::TermList';
use aliased 'AI::Prolog::TermList::Step';
use aliased 'AI::Prolog::TermList::Primitive';
use aliased 'AI::Prolog::KnowledgeBase';
use aliased 'AI::Prolog::Parser';
use aliased 'AI::Prolog::ChoicePoint';

use AI::Prolog::Engine::Primitives;

use constant OnceMark => 'OnceMark';

# The engine is what executes prolog queries.
# Author emeritus:  Dr. Michael Winikoff
# Translation to Perl:  Curtis "Ovid" Poe

# $prog An initial program - this will be extended
# $term The query to be executed

# This governs whether tracing is done
sub trace {
    my $self = shift;
    if (@_) {
        $self->{_trace} = shift;
        return $self;
    }
    return $self->{_trace};
}

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

my $FORMATTED = 1;

sub formatted {
    my $self = shift;
    if (@_) {
        $FORMATTED = shift;
        return $self;
    }
    return $FORMATTED;
}

my $RAW_RESULTS;

sub raw_results {
    my $self = shift;
    if (@_) {
        $RAW_RESULTS = shift;
        if ($RAW_RESULTS) {