sub clone {
    my $self = shift;
    my $new = $self->SUPER::clone( @_ );
    $new->set_ical( $self->get_ical );
    $new;
}

sub clone { 
    my $self = bless { %{ $_[0] } }, ref $_[0];
    $self->{set} = $_[0]->{set}->copy;
    return $self;
}
sub copy {
    my $self = shift;
    my $copy = $self->empty_set();
    ## return $copy unless ref($self);   # constructor!
    foreach my $key (keys %{$self}) {
        if ( ref( $self->{$key} ) eq 'ARRAY' ) {
            @{ $copy->{$key} } = @{ $self->{$key} };
        }
        else {
            $copy->{$key} = $self->{$key};
        }
    }
    return $copy;
}