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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_doc(self, docstring):
 # type: (six.text_type) -> None """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. """

        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