import string


def get_available_letters(letters_guessed):
    """ letters_guessed: list (of letters), which letters have been
        guessed so far
    Returns: string (of letters), comprised of letters that represents
        which letters have not yet been guessed.
    """
    lo, g = string.ascii_lowercase, letters_guessed
    return lo if g == '' else ''.join([c for c in lo if c not in g])