public function add_children(& $node, $parentidnumber) {
        foreach ($this->flat as $competency) {
            if ($competency->parentidnumber == $parentidnumber) {
                $this->progress->increment_progress();
                $node->children[] = $competency;
                $this->add_children($competency, $competency->idnumber);
            }
        }
    }