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diff --git a/math/slalib/drange.f b/math/slalib/drange.f
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+      DOUBLE PRECISION FUNCTION slDA1P (ANGLE)
+*+
+*     - - - - - - -
+*      D A 1 P
+*     - - - - - - -
+*
+*  Normalize angle into range +/- pi  (double precision)
+*
+*  Given:
+*     ANGLE     dp      the angle in radians
+*
+*  The result (double precision) is ANGLE expressed in the range +/- pi.
+*
+*  P.T.Wallace   Starlink   23 November 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 ANGLE
+
+      DOUBLE PRECISION DPI,D2PI
+      PARAMETER (DPI=3.141592653589793238462643D0)
+      PARAMETER (D2PI=6.283185307179586476925287D0)
+
+
+      slDA1P=MOD(ANGLE,D2PI)
+      IF (ABS(slDA1P).GE.DPI)
+     :          slDA1P=slDA1P-SIGN(D2PI,ANGLE)
+
+      END