def is_valid_exit(exits, chosen_exit):
    """This function checks, given a dictionary "exits" (see map.py) and
    a players's choice "chosen_exit" whether the player has chosen a valid exit.
    It returns True if the exit is valid, and False otherwise. Assume that
    the name of the exit has been normalised by the function normalise_input().
    """
    return chosen_exit in exits