ash/amuchastegui    sept 1968     subroutine preces
calculate precession matrix and mean obliquity and also
derivatives of precession w.r.t. time and precession,obliquity
constants

Note that the precession matrix used to calculate the Moon
rotation does not include the ad hoc small rotations used to
rotate Earth sites--this allows separate determination of Earth
and Moon orientation.
partial of precesion matrix with respect
to precession constant (seconds of arc per century)