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+#include "erfa.h"
+
+void eraC2ixys(double x, double y, double s, double rc2i[3][3])
+/*
+** - - - - - - - - - -
+** e r a C 2 i x y s
+** - - - - - - - - - -
+**
+** Form the celestial to intermediate-frame-of-date matrix given the CIP
+** X,Y and the CIO locator s.
+**
+** Given:
+** x,y double Celestial Intermediate Pole (Note 1)
+** s double the CIO locator s (Note 2)
+**
+** Returned:
+** rc2i double[3][3] celestial-to-intermediate matrix (Note 3)
+**
+** Notes:
+**
+** 1) The Celestial Intermediate Pole coordinates are the x,y
+** components of the unit vector in the Geocentric Celestial
+** Reference System.
+**
+** 2) The CIO locator s (in radians) positions the Celestial
+** Intermediate Origin on the equator of the CIP.
+**
+** 3) The matrix rc2i is the first stage in the transformation from
+** celestial to terrestrial coordinates:
+**
+** [TRS] = RPOM * R_3(ERA) * rc2i * [CRS]
+**
+** = RC2T * [CRS]
+**
+** where [CRS] is a vector in the Geocentric Celestial Reference
+** System and [TRS] is a vector in the International Terrestrial
+** Reference System (see IERS Conventions 2003), ERA is the Earth
+** Rotation Angle and RPOM is the polar motion matrix.
+**
+** Called:
+** eraIr initialize r-matrix to identity
+** eraRz rotate around Z-axis
+** eraRy rotate around Y-axis
+**
+** Reference:
+**
+** McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
+** IERS Technical Note No. 32, BKG (2004)
+**
+** Copyright (C) 2013-2016, NumFOCUS Foundation.
+** Derived, with permission, from the SOFA library. See notes at end of file.
+*/
+{
+ double r2, e, d;
+
+
+/* Obtain the spherical angles E and d. */
+ r2 = x*x + y*y;
+ e = (r2 > 0.0) ? atan2(y, x) : 0.0;
+ d = atan(sqrt(r2 / (1.0 - r2)));
+
+/* Form the matrix. */
+ eraIr(rc2i);
+ eraRz(e, rc2i);
+ eraRy(d, rc2i);
+ eraRz(-(e+s), rc2i);
+
+ return;
+
+}
+/*----------------------------------------------------------------------
+**
+**
+** Copyright (C) 2013-2016, 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,
+** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+** POSSIBILITY OF SUCH DAMAGE.
+**
+*/