def used_letters(guess, pvb, lst):
    """
    Checks if the player's guess is in the proverb. Adds it to the
    list of used letters if it's not.
    Assumes everything is uppercase.

    :param guess: the player's guess, a single letter
    :type guess: str
    :param pvb: the proverb
    :type pvb: str
    :param lst: known letters
    :type lst: list
    :return: known letters updated and sorted
    :rtype: list
    """
    if guess not in pvb:
        lst.append(guess)

    return sorted(lst)