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    Select  observations according to their absolute position. A0 and D0 are
    sexagesimal strings (degrees, or hour angle for A0 in case of equatorial
    coordinates). The coordinate system (and its equinox for equatorial sys-
    tem) must be provided as third (and fourth) argument to the option.

    In practice, the option converts (if they are different) the given coor-
    dinates to the system used by the observation currently considered (e.g.
    galactic to equatorial 2000). Then it translates the  absolute  position
    to  relative  position.  Finally, it compares the offset position of the
    observation to this computed reference. SET MATCH is used  as  tolerance
    (same  as  FIND /OFFSET). These operations are repeated for all observa-
    tions in the file.

    Beware this option is less efficient than its  equivalent  FIND  /OFFSET
    (which uses only index elements). When considering absolute coordinates,
    the whole position section must be read for each  observations.  As  the
    Class  Data  Format  does not enforce consistency of the reference posi-
    tions of all the observations, all the conversions described above  must
    be  repeated  for all observations. But if you know your file is consis-
    tent, FIND /OFFSET will always be faster for the same result. For  exam-
    ple,  prefer  FIND  /OFFSET  0 0 instead of FIND /POSITION to select the
    spectra near the reference position.

Gildas manager 2020-04-07