def _numbers_to_letters(num):
    """
    Write base-10 numbers using the alphabet 'qrstuvwxyz', so that they won't be
    normalized away.
    """
    numstr = str(num)
    for digit in '0123456789':
        letter = chr(ord(digit) - ord('0') + ord('q'))
        numstr = numstr.replace(digit, letter)
    return numstr