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
Compute the delta for b - a.  Even/odd and odd/even
     are handled specially, as described above.