how to stop rel=nofollow tags being added by htmlawed?

i see the following code in htmlawed.php:

$C['anti_link_spam'] = (isset($C['anti_link_spam']) && is_array($C['anti_link_spam']) && count($C['anti_link_spam']) == 2 && (empty($C['anti_link_spam'][0]) or hl_regex($C['anti_link_spam'][0])) && (empty($C['anti_link_spam'][1]) or hl_regex($C['anti_link_spam'][1]))) ? $C['anti_link_spam'] : 0;
$C['anti_mail_spam'] = isset($C['anti_mail_spam']) ? $C['anti_mail_spam'] : 0;

which relates to rel=nofollow text being added to links.
can someone explain how to change this so it isn't added at all? 
i have a link to add to a blog which has been given as part of a promotional interaction and the giver does not want nofollow tags in there.