PRECESS_XYZ

       Precess equatorial geocentric rectangular coordinates.

       precess_xyz, x, y, z, equinox1, equinox2

       x,y,z: scalars or vectors giving heliocentric rectangular coordinates
              THESE ARE CHANGED UPON RETURNING.

       EQUINOX1: equinox of input coordinates, numeric scalar
       EQUINOX2: equinox of output coordinates, numeric scalar

       x,y,z are changed upon return

   The equatorial geocentric rectangular coords are converted
      to RA and Dec, precessed in the normal way, then changed
      back to x, y and z using unit vectors.

       Precess 1950 equinox coords x, y and z to 2000.
       IDL> precess_xyz,x,y,z, 1950, 2000

       Written by P. Plait/ACC March 24 1999
          (unit vectors provided by D. Lindler)
       Use /Radian call to PRECESS     W. Landsman     November 2000
       Use two parameter call to ATAN   W. Landsman    June 2001

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