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      SUBROUTINE slPRCE (SYSTEM, EP0, EP1, RA, DC)

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      P R C E
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  Precession - either FK4 (Bessel-Newcomb, pre IAU 1976) or
  FK5 (Fricke, post IAU 1976) as required.

  Given:
     SYSTEM     char   precession to be applied: 'FK4' or 'FK5'
     EP0,EP1    dp     starting and ending epoch
     RA,DC      dp     RA,Dec, mean equator & equinox of epoch EP0

  Returned:
     RA,DC      dp     RA,Dec, mean equator & equinox of epoch EP1

  Called:    slDA2P, slPRBN, slPREC, slDS2C,
             slDMXV, slDC2S

  Notes:

     1)  Lowercase characters in SYSTEM are acceptable.

     2)  The epochs are Besselian if SYSTEM='FK4' and Julian if 'FK5'.
         For example, to precess coordinates in the old system from
         equinox 1900.0 to 1950.0 the call would be:
             CALL slPRCE ('FK4', 1900D0, 1950D0, RA, DC)

     3)  This routine will NOT correctly convert between the old and
         the new systems - for example conversion from B1950 to J2000.
         For these purposes see slFK45, slFK54, slF45Z and
         slF54Z.

     4)  If an invalid SYSTEM is supplied, values of -99D0,-99D0 will
         be returned for both RA and DC.

  P.T.Wallace   Starlink   20 April 1990

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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