Participant Count Report offers 'is not role' criteria - but treats it the same as 'is role'

http://drupal.demo.civicrm.org/civicrm/report/event/participantlist

Filter by role IS attendee - then by role IS NOT Attendee - both are treated the same

In the code there is some funky field specific handling - I'm not sure why since I think there are other fields using the pattern participant_role_id uses so I would have thought there would be some generic handling for it. Actually that's not quite true - I believe that field breaks the rules & stores values like

1SEPARATOR2

rather than the normal
SEPARATOR1SEPARATOR2SEPARATOR

Without the special handling for this field the where function on that report is equivalent to

function where() {
$this->_whereClauses[] = "{$this->_aliases['civicrm_participant']}.is_test = 0";
parent::where();
}

NB - I'm not working on this - I'm working on a custom report but I picked this up in the process as I tried to link to this report