copy_subtree

        High-level api: Copy element as a subtree 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_subtree(ret, element, msg):
        '''copy_subtree

        High-level api: Copy element as a subtree 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 = ModelDiff.process_attrib(deepcopy(element), msg)
        ret.append(sub_element)
        return sub_element