$pg->add_fact( gnarfle => qw/ foo bar baz / );
$pg->add_fact( gnarfle => qw/ foo tac toe / );

##</code><code>##

           {
            '2' => [
                     undef,
                     {
                       '3' => 3
                     },
                     {
                       '4' => 4
                     }
                   ]
          }

##</code><code>##

          {
            '2' => []
          }