def apply_raw_r_assimilation(last_syllable: str) ->str:
    """
    -r preceded by an -s-, -l- or -n- becomes respectively en -s, -l or -n.

    >>> apply_raw_r_assimilation("arm")
    'armr'
    >>> apply_raw_r_assimilation("ás")
    'áss'
    >>> apply_raw_r_assimilation("stól")
    'stóll'
    >>> apply_raw_r_assimilation("stein")
    'steinn'
    >>> apply_raw_r_assimilation("vin")
    'vinn'


    :param last_syllable: last syllable of an Old Norse word
    :return:
    """
    if len(last_syllable) > 0:
        if last_syllable[-1] == 'l':
            return last_syllable + 'l'
        elif last_syllable[-1] == 's':
            return last_syllable + 's'
        elif last_syllable[-1] == 'n':
            return last_syllable + 'n'
    return last_syllable + 'r'
