def explain_au(fsm, state, phi, psi, states=None):
    """
    Explain why state of fsm satisfies A phi U psi.
    
    fsm -- the fsm;
    state -- a state of fsm satisfying A phi U psi;
    phi -- the set of states of fsm satifying phi;
    psi -- the set of states of fsm satifying psi;
    states -- a dictionary of state->explanation pairs.
    """
    pass
