public function groupIsMember($group_dn, $user, $nested = NULL) {

    $nested = ($nested === TRUE || $nested === FALSE) ? $nested : $this->groupNested;
    $group_dns = $this->groupMembershipsFromUser($user, 'group_dns', $nested);
    // while list of group dns is going to be in correct mixed case, $group_dn may not since it may be derived from user entered values
    // so make sure in_array() is case insensitive
    return (is_array($group_dns) && in_array(drupal_strtolower($group_dn), $this->dnArrayToLowerCase($group_dns)));
  }