code: def _GetMountpointBlacklist(xdev):
    
    if xdev == rdf_file_finder.FileFinderArgs.XDev.NEVER:
      # Never cross device boundaries, stop at all mount points.
      return _GetMountpoints(only_physical=False)
  
    if xdev == rdf_file_finder.FileFinderArgs.XDev.LOCAL:
      # Descend into file systems on physical devices only.
      physical = _GetMountpoints(only_physical=True)
      return _GetMountpoints(only_physical=False) - physical
  
    if xdev == rdf_file_finder.FileFinderArgs.XDev.ALWAYS:
      # Never stop at any device boundary.
      return set()
  
    raise ValueError("Incorrect `xdev` value: %s" % xdev)
docs: Builds a list of mountpoints to ignore during recursive searches.
  
    Args:
      xdev: A `XDev` value that determines policy for crossing device boundaries.
  
    Returns:
      A set of mountpoints to ignore.
  
    Raises:
      ValueError: If `xdev` value is invalid.