 =

      followed by a comma-separated list of one or more surface
      identifiers. The identifiers may be names or integer codes in
      string format. For example, suppose we have the surface
      names and corresponding ID codes shown below:

         Surface Name                              ID code
         ------------                              -------
         'Mars MEGDR 128 PIXEL/DEG'                1
         'Mars MEGDR 64 PIXEL/DEG'                 2
         'Mars_MRO_HIRISE'           