text: #include "erfa.h"
  
  double eraS00b(double date1, double date2)
  /*
  **  - - - - - - - -
  **   e r a S 0 0 b
  **  - - - - - - - -
  **
  **  The CIO locator s, positioning the Celestial Intermediate Origin on
  **  the equator of the Celestial Intermediate Pole, using the IAU 2000B
  **  precession-nutation model.
  **
  **  Given:
  **     date1,date2  double    TT as a 2-part Julian Date (Note 1)
  **
  **  Returned (function value):
  **                  double    the CIO locator s in radians (Note 2)
  **
  **  Notes:
  **
  **  1) The TT date date1+date2 is a Julian Date, apportioned in any
  **     convenient way between the two arguments.  For example,
  **     JD(TT)=2450123.7 could be expressed in any of these ways,
  **     among others:
  **
  **            date1          date2
  **
  **         2450123.7           0.0       (JD method)
  **         2451545.0       -1421.3       (J2000 method)
  **         2400000.5       50123.2       (MJD method)
  **         2450123.5           0.2       (date & time method)
  **
  **     The JD method is the most natural and convenient to use in
  **     cases where the loss of several decimal digits of resolution
  **     is acceptable.  The J2000 method is best matched to the way
  **     the argument is handled internally and will deliver the
  **     optimum resolution.  The MJD method and the date & time methods
  **     are both good compromises between resolution and convenience.
  **
  **  2) 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 CIO locator s remains a small fraction of
  **     1 arcsecond throughout 1900-2100.
  **
  **  3) The series used to compute s is in fact for s+XY/2, where X and Y
  **     are the x and y components of the CIP unit vector;  this series
  **     is more compact than a direct series for s would be.  The present
  **     function uses the IAU 2000B truncated nutation model when
  **     predicting the CIP position.  The function eraS00a uses instead
  **     the full IAU 2000A model, but with no significant increase in
  **     accuracy and at some cost in speed.
  **
  **  Called:
  **     eraPnm00b    classical NPB matrix, IAU 2000B
  **     eraBnp2xy    extract CIP X,Y from the BPN matrix
  **     eraS00       the CIO locator s, given X,Y, IAU 2000A
  **
  **  References:
  **
  **     Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
  **     "Expressions for the Celestial Intermediate Pole and Celestial
  **     Ephemeris Origin consistent with the IAU 2000A precession-
  **     nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
  **
  **     n.b. The celestial ephemeris origin (CEO) was renamed "celestial
  **          intermediate origin" (CIO) by IAU 2006 Resolution 2.
  **
  **     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
  **     IERS Technical Note No. 32, BKG (2004)
  **
  **  Copyright (C) 2013-2017, NumFOCUS Foundation.
  **  Derived, with permission, from the SOFA library.  See notes at end of file.
  */
  {
     double rbpn[3][3], x, y, s;
  
  
  /* Bias-precession-nutation-matrix, IAU 2000B. */
     eraPnm00b(date1, date2, rbpn);
  
  /* Extract the CIP coordinates. */
     eraBpn2xy(rbpn, &x, &y);
  
  /* Compute the CIO locator s, given the CIP coordinates. */
     s = eraS00(date1, date2, x, y);
  
     return s;
  
  }
  /*----------------------------------------------------------------------
  **  
  **  
  **  Copyright (C) 2013-2017, NumFOCUS Foundation.
  **  All rights reserved.
  **  
  **  This library is derived, with permission, from the International
  **  Astronomical Union's "Standards of Fundamental Astronomy" library,
  **  available from http://www.iausofa.org.
  **  
  **  The ERFA version is intended to retain identical functionality to
  **  the SOFA library, but made distinct through different function and
  **  file names, as set out in the SOFA license conditions.  The SOFA
  **  original has a role as a reference standard for the IAU and IERS,
  **  and consequently redistribution is permitted only in its unaltered
  **  state.  The ERFA version is not subject to this restriction and
  **  therefore can be included in distributions which do not support the
  **  concept of "read only" software.
  **  
  **  Although the intent is to replicate the SOFA API (other than
  **  replacement of prefix names) and results (with the exception of
  **  bugs;  any that are discovered will be fixed), SOFA is not
  **  responsible for any errors found in this version of the library.
  **  
  **  If you wish to acknowledge the SOFA heritage, please acknowledge
  **  that you are using a library derived from SOFA, rather than SOFA
  **  itself.
  **  
  **  
  **  TERMS AND CONDITIONS
  **  
  **  Redistribution and use in source and binary forms, with or without
  **  modification, are permitted provided that the following conditions
  **  are met:
  **  
  **  1 Redistributions of source code must retain the above copyright
  **    notice, this list of conditions and the following disclaimer.
  **  
  **  2 Redistributions in binary form must reproduce the above copyright
  **    notice, this list of conditions and the following disclaimer in
  **    the documentation and/or other materials provided with the
  **    distribution.
  **  
  **  3 Neither the name of the Standards Of Fundamental Astronomy Board,
  **    the International Astronomical Union nor the names of its
  **    contributors may be used to endorse or promote products derived
  **    from this software without specific prior written permission.
  **  
  **  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  **  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  **  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  **  FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  **  COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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  */
  