code: def invoke(self, ns, request_name, params={}, simplify=False):
          
          if self.auth_token == None:
              raise AuthException('Unable to invoke zimbra method')
  
          if util.empty(request_name):
              raise ZimbraClientException('Invalid request')
  
          return self.transport.invoke(ns,
                                       request_name,
                                       params,
                                       self.auth_token,
                                       simplify)
docs: Invokes zimbra method using established authentication session.
          @param req: zimbra request
          @parm params: request params
          @param simplify: True to return python object, False to return xml struct
          @return: zimbra response
          @raise AuthException: if authentication fails
          @raise SoapException: wrapped server exception