1) The CIO locator s is the difference between the right ascensions
of the same point in two systems:  the two systems are the GCRS
and the CIP,CIO, and the point is the ascending node of the
CIP equator.  The quantity s remains below 0.1 arcsecond
throughout 1900-2100.

This routine is based on the International Astronomical Union's
SOFA (Standards of Fundamental Astronomy) software collection,
but it is not itself part of the SOFA collection.  It has been
extracted from function iau_S06 and modified to return directly
the composite extended-precision value of s+XY/2.  It is also
modified to use the constants encoded globally in PEP, but it
calls the original SOFA routines for the fundamental arguments
used to evaluate the series for s+XY/2.

Called routines from the SOFA collection:
iau_FAL03    mean anomaly of the Moon
iau_FALP03   mean anomaly of the Sun
iau_FAF03    mean argument of the latitude of the Moon
iau_FAD03    mean elongation of the Moon from the Sun
iau_FAOM03   mean longitude of the Moon's ascending node
iau_FAVE03   mean longitude of Venus
iau_FAE03    mean longitude of Earth
iau_FAPA03   general accumulated precession in longitude