docs: def _AddDelta(a, delta):
    """Return a + delta, handling EvenOdd and OddEven specially."""
    if type(delta) == int:
      return a+delta
    if delta == 'EvenOdd':
      if a%2 == 0:
        return a+1
      else:
        return a-1
    if delta == 'OddEven':
      if a%2 == 1:
        return a+1
      else:
        return a-1
    print >>sys.stderr, "Bad Delta: ", delta
    raise "Bad Delta"