sub insert_after {
	my ($previous, $val) = @_;
	my $curr_next = $previous->{next};
	$previous->{next} = {
                "data" => $val , 
                "next" => $curr_next,
                "prev" => $previous,
            };
    my $new_next = $previous->{next};
    $curr_next->{prev} = $new_next if defined $curr_next;
    return $new_next;
}	

sub insert_before {
	# simply call insert_after on the previous node
	my ($previous, $val) = @_;
	return insert_after($previous->{prev}, $val);
}