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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 then_by(self, key_selector=identity):
        '''Introduce subsequent ordering to the sequence with an optional key.

        The returned sequence will be sorted in ascending order by the
        selected key.

        Note: This method uses deferred execution.

        Args:
            key_selector: A unary function the only positional argument to
                which is the element value from which the key will be
                selected.  The return value should be the key from that
                element.

        Returns:
            An OrderedQueryable over the sorted items.

        Raises:
            ValueError: If the OrderedQueryable is closed().
            TypeError: If key_selector is not callable.
        '''
        if self.closed():
            raise ValueError("Attempt to call then_by() on a "
                             "closed OrderedQueryable.")

        if not is_callable(key_selector):
            raise TypeError("then_by() parameter key_selector={key_selector} "
                    "is not callable".format(key_selector=repr(key_selector)))

        self._funcs.append((-1, key_selector))
        return self