code: def _plain_auth_stage2(self, _unused):
          
          iq=Iq(stanza_type="set")
          q=iq.new_query("jabber:iq:auth")
          q.newTextChild(None,"username",to_utf8(self.my_jid.node))
          q.newTextChild(None,"resource",to_utf8(self.my_jid.resource))
          q.newTextChild(None,"password",to_utf8(self.password))
          self.send(iq)
          self.set_response_handlers(iq,self.auth_finish,self.auth_error)
          iq.free()
docs: Do the second stage (<iq type='set'/>) of legacy "plain"
          authentication.
  
          [client only]