use strict;
use warnings;
Package Company;
sub new {
    my $self = shift;
    my $regno = shift or die "Must supply reg. no.";
    $self = {};
    $self -> { REGNO } = $regno;
    return bless $self;
}
sub parent {
    my $self = shift;
    my $parent;
    unless ( $parent = shift() ) {
        if (defined $self -> { PARENT } ) {
            return $self -> { PARENT };
        else {
            return undef();
        }
    }
    if ( defined $parent -> { REGNO } ) {
        $self -> { PARENT } = $parent;
    }
    else {
        die "Invalid parent reference";
    }