docs: def _get_generality(key, mask):
      """Count the number of Xs in the key-mask pair.
  
      For example, there are 32 Xs in ``0x00000000/0x00000000``::
  
          >>> _get_generality(0x0, 0x0)
          32
  
      And no Xs in ``0xffffffff/0xffffffff``::
  
          >>> _get_generality(0xffffffff, 0xffffffff)
          0
      """
      xs = (~key) & (~mask)
      return sum(1 for i in range(32) if xs & (1 << i))