def neighbours(guest, plan):
    """Return guests’s adjacent neighbours in plan

    Guests at one end of the plan are considered to be sitting next
    to the guest at the opposite end.
    """
    g_index = plan.index(guest)
    left = plan[g_index - 1]
    right = plan[(g_index + 1) % len(plan)]  # Wrap around to zero
    return (left, right)