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diff --git a/math/slalib/dtp2s.f b/math/slalib/dtp2s.f
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+      SUBROUTINE slDTPS (XI, ETA, RAZ, DECZ, RA, DEC)
+*+
+*     - - - - - -
+*      D T P S
+*     - - - - - -
+*
+*  Transform tangent plane coordinates into spherical
+*  (double precision)
+*
+*  Given:
+*     XI,ETA      dp   tangent plane rectangular coordinates
+*     RAZ,DECZ    dp   spherical coordinates of tangent point
+*
+*  Returned:
+*     RA,DEC      dp   spherical coordinates (0-2pi,+/-pi/2)
+*
+*  Called:        slDA2P
+*
+*  P.T.Wallace   Starlink   24 July 1995
+*
+*  Copyright (C) 1995 Rutherford Appleton Laboratory
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 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 General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the
+*    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+*    Boston, MA  02110-1301  USA
+*
+*  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION XI,ETA,RAZ,DECZ,RA,DEC
+
+      DOUBLE PRECISION slDA2P
+
+      DOUBLE PRECISION SDECZ,CDECZ,DENOM
+
+
+
+      SDECZ=SIN(DECZ)
+      CDECZ=COS(DECZ)
+
+      DENOM=CDECZ-ETA*SDECZ
+
+      RA=slDA2P(ATAN2(XI,DENOM)+RAZ)
+      DEC=ATAN2(SDECZ+ETA*CDECZ,SQRT(XI*XI+DENOM*DENOM))
+
+      END