def expand_basic(state):
    """
    Simple function which returns child states by appending an available move to 
    current state.
    """
    assert(len(state) < 9)

    # Calculte set difference to get remaining moves.
    n = tuple(set(range(9)) - set(state))

    # Create tuple of available new states and return to caller.
    c = tuple(state + (q,) for q in n)
    return c