Form the matrix of precession and nutation (IAU 1976 / FK5) (double precision)
epoch double Julian epoch for mean coordinates date double Modified Julian Date (JD-2400000.5) for true coordinates
rmatpn double[3][3] combined precession/nutation matrix
slaPrec, slaEpj, slaNut, slaDmxm
1) The epoch and date are TDB (loosely ET). 2) The matrix is in the sense v(true) = rmatpn * v(mean) . P.T.Wallace Starlink 15 July 1993