code: def add_doc(self, docstring):
          # type: (six.text_type) -> None
          
          assert isinstance(docstring, six.text_type), type(docstring)
          normalized_docstring = doc_unwrap(docstring) + '\n'
          if self.doc is None:
              self.doc = normalized_docstring
          else:
              self.doc += normalized_docstring
docs: Adds a docstring for this namespace.
  
          The input docstring is normalized to have no leading whitespace and
          no trailing whitespace except for a newline at the end.
  
          If a docstring already exists, the new normalized docstring is appended
          to the end of the existing one with two newlines separating them.