This function converts stars from the Bessel-Newcomb, FK4 system to the IAU 1976, FK5, Fricke system, in such a way that the FK5 proper motion is zero. Because such a star has, in general, a non-zero proper motion in the FK4 system, the routine requires the epoch at which the position in the FK4 system was determined.
The method is from Appendix 2 of Ref 1, but using the constants of Ref 4.