docs: def update_default_values(self, **updates):
          """Replace the default values of specified fields.
  
          Parameters
          ----------
          Parameters are taken as keyword-arguments of `field=new_value`.
  
          Raises
          ------
          KeyError
              If a field doesn't exist in the struct.
          """
          for (field, value) in six.iteritems(updates):
              fname = six.b(field)
              self[fname] = self[fname]._replace(default=value)