XYXY

       To use a pair of headers to convert X/Y positions from one frame
       to another.

       XYXY, hdra, hdrb, xa, ya, [ xb, yb ]

       hdra - The header containing the plate solution describing the
              frame of reference being converted FROM.
       hdra - The header containing the plate solution describing the
              frame of reference being converted TO.
       xa   - A scalar or vector containing the x coordinate(s) to convert.
       ya   - A scalar or vector containing the y coordinate(s) to convert.
              Must have the same number of elements as 'xa'.

       xb   - The converted x coordinate(s).  If this parameter is not
              specified, it is returned through 'xa'.
       yb   - The converted y coordinate(s).  If this parameter is not
              specified, it is returned through 'ya'.

       The procedures 'xyad' and 'adxy' are used to perform the
       conversion.     The equinoxes of each header are checked with
       "get_equinox" to make sure that they are identical, and "precess"
       is used if they are not.   HEULER used if the headers have a different
       coordinate system (e.g. Celestial, Galactic, Ecliptic)
       Note that all X,Y coordinates are in the IDL convention (starting with
       0,0) and not the FITS convention (first pixel is 1,1)

       GET_EQUINOX(), EXTAST, XYAD, ADXY, PRECESS, HEULER

       Written by Michael R. Greason, Hughes-STX, 13 April 1992.
       Updated to use ASTROMETRY structures.  J.D.Offenberg, HSTX, Jan 1993
       Converted to IDL V5.0   W. Landsman   September 1997
       Check coordinate system   J. Ballet/ W. Landsman  April 2004

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