def METHOD_NAME(cls, node, runtime, _keys, _id_generator):
    """
    Transforms the xml_data from <$custom_tag attr="" attr=""/> to
    <customtag attr="" attr="" impl="$custom_tag"/>
    """
    runtime.error_tracker(Text('WARNING: the <{tag}> tag is deprecated.  '
                          'Instead, use <customtag impl="{tag}" attr1="..." attr2="..."/>. ')
                          .format(tag=node.tag))
    tag = node.tag
    node.tag = 'customtag'
    node.attrib['impl'] = tag
    return runtime.process_xml(etree.tostring(node))