<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def states(self):
 """ All valid states; "Q" in the mathematical description of a state machine. """

        return frozenset(
            chain.from_iterable(
                (inState, outState)
                for
                (inState, inputSymbol, outState, outputSymbol)
                in self._transitions
            )
        )