owns( merlyn, gold ).
owns( ovid, pocketLint ).
owns( ovid, cheapWhiskey ).
owns( kudra, dominationFund ).

valuable( gold ).
valuable( dominationFund ).

thief( badguy ).

rich( X ) :-
  owns( X, Y ),
  valuable( Y ).

steals_from( X, Y ) :-
  thief( X ),
  rich( Y ).

##</code><code>##

badguy will steal from merlyn
badguy will not steal from ovid
badguy will steal from kudra

##</code><code>##

#!/usr/bin/perl -w
use strict;

my %facts;

# poor ( X ) :-
#   not rich( X ). 

my @owns = (
  2,
  merlyn => 'gold',
  ovid   => 'pocket lint',
  ovid   => 'cheap whiskey',
  kudra  => 'domination fund'
);

my @valuable = (
  1,
  'gold',
  'domination fund'
);

my @thief = (
  1,
  'badguy'
);

$facts{ owns     } =  add_fact( \@owns     );
$facts{ valuable } =  add_fact( \@valuable );
$facts{ thief    } =  add_fact( \@thief    );

foreach ( qw/ merlyn ovid kudra / ) {
  my $modifier = steals_from( 'badguy', $_ ) ? '' : ' not';
  print "badguy will$modifier steal from $_\n";
}

sub steals_from {
  # steals_from( X, Y ) :-
  #   thief( X ),
  #   rich( Y ).
  my ( $perp, $victim ) = @_;
  return thief( $perp ) && rich( $victim );
}

sub thief {
  my $perp = shift;
  return grep { $_->[0] eq $perp } @{$facts{thief}};
}

sub rich {
  # rich ( X ) :-
  #  owns( X, Y ),
  #  valuable( Y ).
  my $person = shift;
  my @results;
  my @items = 
   map  { $_->[1] }
   grep { $_->[0] eq $person } 
     @{$facts{owns}};
  foreach my $item ( @items ) {
    push @results => $item
      if grep { $_->[0] eq $item } @{$facts{valuable}};
  }
  return @results ? 1 : 0;
}

sub add_fact {
  my $rule = shift;
  my $args = shift @$rule;
  my @rule;
 
  for ( my $i = 0; $i < @$rule; $i = $i + $args ) {
    my @args = @$rule[ $i .. $i + ( $args - 1 ) ];
    push @rule => \@args;
  }
  return \@rule;
}

##</code><code>##

(?(?{attack $q [3], $q [4]})x|)