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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_alt_text(self, alt_text):
 """Adds an alt_text. arg: alt_text (displayText):
 the new alt_text raise: InvalidArgument - ``alt_text`` is invalid raise: NoAccess - ``Metadata.isReadOnly()`` is ``true`` raise: NullArgument - ``alt_text`` is ``null`` *compliance: mandatory -- This method must be implemented.* """

        if self.get_alt_texts_metadata().is_read_only():
            raise NoAccess()
        self.add_or_replace_value('altTexts', alt_text)