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diff --git a/ast/pal/palDtp2s.c b/ast/pal/palDtp2s.c
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+/*
+*+
+*  Name:
+*     palDtp2s
+
+*  Purpose:
+*     Tangent plane to spherical coordinates
+
+*  Language:
+*     Starlink ANSI C
+
+*  Type of Module:
+*     Library routine
+
+*  Invocation:
+*     palDtp2s( double xi, double eta, double raz, double decz,
+*               double *ra, double *dec);
+
+*  Arguments:
+*     xi = double (Given)
+*        First rectangular coordinate on tangent plane (radians)
+*     eta = double (Given)
+*        Second rectangular coordinate on tangent plane (radians)
+*     raz = double (Given)
+*        RA spherical coordinate of tangent point (radians)
+*     decz = double (Given)
+*        Dec spherical coordinate of tangent point (radians)
+*     ra = double * (Returned)
+*        RA spherical coordinate of point to be projected (radians)
+*     dec = double * (Returned)
+*        Dec spherical coordinate of point to be projected (radians)
+
+*  Description:
+*     Transform tangent plane coordinates into spherical.
+
+*  Authors:
+*     PTW: Pat Wallace (STFC)
+*     TIMJ: Tim Jenness (JAC, Hawaii)
+*     {enter_new_authors_here}
+
+*  History:
+*     2012-02-08 (TIMJ):
+*        Initial version with documentation taken from Fortran SLA
+*        Adapted with permission from the Fortran SLALIB library.
+*     {enter_further_changes_here}
+
+*  Copyright:
+*     Copyright (C) 1995 Rutherford Appleton Laboratory
+*     Copyright (C) 2012 Science and Technology Facilities Council.
+*     All Rights Reserved.
+
+*  Licence:
+*     This program is free software: you can redistribute it and/or
+*     modify it under the terms of the GNU Lesser General Public
+*     License as published by the Free Software Foundation, either
+*     version 3 of the License, or (at your option) any later
+*     version.
+*
+*     This program is distributed in the hope that it will be useful,
+*     but WITHOUT ANY WARRANTY; without even the implied warranty of
+*     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*     GNU Lesser General Public License for more details.
+*
+*     You should have received a copy of the GNU Lesser General
+*     License along with this program.  If not, see
+*     <http://www.gnu.org/licenses/>.
+
+*  Bugs:
+*     {note_any_bugs_here}
+*-
+*/
+
+#include "pal.h"
+#include "pal1sofa.h"
+
+#include <math.h>
+
+void
+palDtp2s ( double xi, double eta, double raz, double decz,
+           double *ra, double *dec ) {
+
+  double cdecz;
+  double denom;
+  double sdecz;
+  double d;
+
+  sdecz = sin(decz);
+  cdecz = cos(decz);
+  denom = cdecz - eta * sdecz;
+  d = atan2(xi, denom) + raz;
+  *ra = eraAnp(d);
+  *dec = atan2(sdecz + eta * cdecz, sqrt(xi * xi + denom * denom));
+
+  return;
+}