um          = 0;
		$parent_field = $this->db_fields['parent'];

		foreach ( $elements as $e ) {
			if ( empty( $e->$parent_field ) ) {
				$num++;
			}
		}
		return $num;
	}

	/**
	 * Unsets all the children for a given top level element.
	 *
	 * @since 2.7.0
	 *
	 * @param object $element           The top level element.
	 * @param array  $children_elements The children elements.
	 */
	public function unset_children( $element, &$children_elements ) {
		if ( ! $element || ! $children_elements ) {
			return;
		}

		$id_field = $this->db_fields['id'];
		$id       = $element->$id_field;

		if ( ! empty( $children_elements[ $id ] ) && is_array( $children_elements[ $id ] ) ) {
			foreach ( (array) $children_elements[ $id ] as $child ) {
				$this->unset_children( $child, $children_elements );
			}
		}

		unset( $children_elements[ $id ] );
	}

}