<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tostring(self):
 """return a pretty-printed string output for rpc reply"""

        parser = etree.XMLParser(remove_blank_text=True)
        outputtree = etree.XML(etree.tostring(self.__doc), parser)
        return etree.tostring(outputtree, pretty_print=True)