<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def RegisterMountPoint(cls, mount_point, path_spec):
 """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. """

    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