def downcase_word(event):
    """
    Lowercase the current (or following) word.
    """
    buff = event.current_buffer

    for i in range(event.arg):  # XXX: not DRY: see meta_c and meta_u!!
        pos = buff.document.find_next_word_ending()
        words = buff.document.text_after_cursor[:pos]
        buff.insert_text(words.lower(), overwrite=True)
Lowercase the current (or following) word.