copy_node

        High-level api: Copy element as a node without its children and put it
        as a child of ret.

        Parameters
        ----------

        element : `Element`
            A node in a model tree.

        msg : `str`
            Message to be added.

        ret : `Element`
            A node in self.tree.

        Returns
        -------

        None
            Nothing returns.

def copy_node(ret, element, msg):
        '''copy_node

        High-level api: Copy element as a node without its children and put it
        as a child of ret.

        Parameters
        ----------

        element : `Element`
            A node in a model tree.

        msg : `str`
            Message to be added.

        ret : `Element`
            A node in self.tree.

        Returns
        -------

        None
            Nothing returns.
        '''
        sub_element = etree.SubElement(ret, element.tag, attrib=element.attrib)
        ModelDiff.process_attrib(sub_element, msg)
        return sub_element