Return the `c:tx[c:rich]` subtree, newly created if not present.

def get_or_add_tx_rich(self):
        """
        Return the `c:tx[c:rich]` subtree, newly created if not present.
        """
        tx = self.get_or_add_tx()
        tx._remove_strRef()
        tx.get_or_add_rich()
        return tx