docs: def default(self, value):
          """Change of resolver name.
  
          :param value: new default value to use.
          :type value: str or callable
          :raises: KeyError if value is a string not already registered."""
  
          if value is None:
              if self:
                  value = list(self.keys())[0]
  
          elif not isinstance(value, string_types):
              value = register(exprresolver=value, reg=self)
  
          elif value not in self:
              raise KeyError(
                  '{0} not registered in {1}'.format(value, self)
              )
  
          self._default = value