def normalize_skin_tone(tone):
    """
    Converts from the more visual skin tone preferences string to a more
    machine-readable format.
    """
    if tone == "👌 default": return ''
    elif tone == "👌🏻 light": return 'light'
    elif tone == "👌🏼 medium-light": return 'medium-light'
    elif tone == "👌🏽 medium": return 'medium'
    elif tone == "👌🏾 medium-dark": return 'medium-dark'
    elif tone == "👌🏿 dark": return 'dark'
    else: return None