docs: def _Delta(a, b):
    """Compute the delta for b - a.  Even/odd and odd/even
       are handled specially, as described above."""
    if a+1 == b:
      if a%2 == 0:
        return 'EvenOdd'
      else:
        return 'OddEven'
    if a == b+1:
      if a%2 == 0:
        return 'OddEven'
      else:
        return 'EvenOdd'
    return b - a