PREMAT

       Return the precession matrix needed to go from EQUINOX1 to EQUINOX2.

       This matrix is used by the procedures PRECESS and BARYVEL to precess
       astronomical coordinates

       matrix = PREMAT( equinox1, equinox2, [ /FK4 ] )

       EQUINOX1 - Original equinox of coordinates, numeric scalar.
       EQUINOX2 - Equinox of precessed coordinates.

      matrix - double precision 3 x 3 precession matrix, used to precess
               equatorial rectangular coordinates

       /FK4   - If this keyword is set, the FK4 (B1950.0) system precession
               angles are used to compute the precession matrix.   The
               default is to use FK5 (J2000.0) precession angles

       Return the precession matrix from 1950.0 to 1975.0 in the FK4 system
       IDL> matrix = PREMAT( 1950.0, 1975.0, /FK4)

       FK4 constants from "Computational Spherical Astronomy" by Taff (1983),
       p. 24. (FK4). FK5 constants from "Astronomical Almanac Explanatory
       Supplement 1992, page 104 Table 3.211.1.

       Written, Wayne Landsman, HSTX Corporation, June 1994
       Converted to IDL V5.0   W. Landsman   September 1997

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