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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _GetMountpointBlacklist(xdev):
 """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. """

  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)