@staticmethod<EOL><INDENT>def copy_subtree(ret, element, msg):<DEDENT>
sub_element = ModelDiff.process_attrib(deepcopy(element), msg)<EOL>ret.append(sub_element)<EOL>return sub_element<EOL>
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.