docs: def segmentAcceptable(RCV_NXT, RCV_WND, SEG_SEQ, SEG_LEN):
      """
      An acceptable segment: RFC 793 page 26.
      """
      if SEG_LEN == 0 and RCV_WND == 0:
          return SEG_SEQ == RCV_NXT
      if SEG_LEN == 0 and RCV_WND > 0:
          return ((RCV_NXT <= SEG_SEQ) and (SEG_SEQ < RCV_NXT + RCV_WND))
      if SEG_LEN > 0 and RCV_WND == 0:
          return False
      if SEG_LEN > 0 and RCV_WND > 0:
          return ((  (RCV_NXT <= SEG_SEQ) and (SEG_SEQ < RCV_NXT + RCV_WND))
                  or ((RCV_NXT <= SEG_SEQ+SEG_LEN-1) and
                      (SEG_SEQ+SEG_LEN-1 < RCV_NXT + RCV_WND)))
      assert 0, 'Should be impossible to get here.'
      return False