code: def add_alt_text(self, alt_text):
          
          if self.get_alt_texts_metadata().is_read_only():
              raise NoAccess()
          self.add_or_replace_value('altTexts', alt_text)
docs: 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.*