code: def fields(*fields, **keys):
      
      return list(fields) + [
          Field.forTypes(key, [value], "") for key, value in keys.items()]
docs: Factory for for L{MessageType} and L{ActionType} field definitions.
  
      @param *fields: A L{tuple} of L{Field} instances.
  
      @param **keys: A L{dict} mapping key names to the expected type of the
          field's values.
  
      @return: A L{list} of L{Field} instances.