public function structureProto()
    {
        if ($this->structurePrototype === null) {
            $model = $this->createStructureModel();

            if ($model instanceof DescribableInterface) {
                $model->setMetadata($this->structureMetadata());
            }

            $this->structurePrototype = $model;
        }

        return $this->structurePrototype;
    }