def convert_tree(self, element1, element2=None):
        '''convert_tree

        High-level api: Convert cxml tree to an internal schema tree. This
        method is recursive.

        Parameters
        ----------

        element1 : `Element`
            The node to be converted.

        element2 : `Element`
            A new node being constructed.

        Returns
        -------

        Element
            This is element2 after convertion.
        '''

        if element2 is None:
            attributes = deepcopy(element1.attrib)
            tag = attributes['name']
            del attributes['name']
            element2 = etree.Element(tag, attributes)
        for e1 in element1.findall('node'):
            attributes = deepcopy(e1.attrib)
            tag = self.prefix_to_url(attributes['name'])
            del attributes['name']
            e2 = etree.SubElement(element2, tag, attributes)
            self.convert_tree(e1, e2)
        return element2