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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 DeregisterMountPoint(cls, mount_point):
 """Deregisters a path specification mount point. Args: mount_point (str):
 mount point identifier. Raises: KeyError: if the corresponding mount point is not set. """

    if mount_point not in cls._mount_points:
      raise KeyError('Mount point: {0:s} not set.'.format(mount_point))

    del cls._mount_points[mount_point]