code: def validate(self):
          
          if not (isinstance(self.start_class, set) and
                  all(isinstance(x, six.string_types) for x in self.start_class)):
              raise TypeError(u'Expected set of string start_class, got: {} {}'.format(
                  type(self.start_class).__name__, self.start_class))
  
          for cls in self.start_class:
              validate_safe_string(cls)
docs: Ensure that the QueryRoot block is valid.