public function setGroupsToDisplay($groups_to_display)
    {
        if (!is_array($groups_to_display)) {
            throw new TypeException('$groups_to_display must be an array.');
        }
        if (is_array($groups_to_display)) {
            $this->groups_to_display = $groups_to_display;
        } else {
            $this->groups_to_display[] = $groups_to_display;
        }
    }