docs: def get_common_xs(entries):
      """Return a mask of where there are Xs in all routing table entries.
  
      For example ``01XX`` and ``XX1X`` have common Xs in the LSB only, for this
      input this method would return ``0b0001``::
  
          >>> from rig.routing_table import RoutingTableEntry
          >>> entries = [
          ...     RoutingTableEntry(set(), 0b0100, 0xfffffff0 | 0b1100),  # 01XX
          ...     RoutingTableEntry(set(), 0b0010, 0xfffffff0 | 0b0010),  # XX1X
          ... ]
          >>> print("{:#06b}".format(get_common_xs(entries)))
          0b0001
      """
      # Determine where there are never 1s in the key and mask
      key = 0x00000000
      mask = 0x00000000
  
      for entry in entries:
          key |= entry.key
          mask |= entry.mask
  
      # Where there are never 1s in the key or the mask there are Xs which are
      # common to all entries.
      return (~(key | mask)) & 0xffffffff