code: def RegisterMountPoint(cls, mount_point, path_spec):
      
      if mount_point in cls._mount_points:
        raise KeyError('Mount point: {0:s} already set.'.format(mount_point))
  
      cls._mount_points[mount_point] = path_spec
docs: Registers a path specification mount point.
  
      Args:
        mount_point (str): mount point identifier.
        path_spec (PathSpec): path specification of the mount point.
  
      Raises:
        KeyError: if the corresponding mount point is already set.