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      SUBROUTINE slPRCS (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     int    precession to be applied: 1 = FK4 or 2 = 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 slPRCS (1, 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

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