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+      DOUBLE PRECISION FUNCTION slDSEPV (V1, V2)
+*+
+*     - - - - - -
+*      D S E P V
+*     - - - - - -
+*
+*  Angle between two vectors.
+*
+*  (double precision)
+*
+*  Given:
+*     V1      d(3)    first vector
+*     V2      d(3)    second vector
+*
+*  The result is the angle, in radians, between the two vectors.  It
+*  is always positive.
+*
+*  Notes:
+*
+*  1  There is no requirement for the vectors to be unit length.
+*
+*  2  If either vector is null, zero is returned.
+*
+*  3  The simplest formulation would use dot product alone.  However,
+*     this would reduce the accuracy for angles near zero and pi.  The
+*     algorithm uses both cross product and dot product, which maintains
+*     accuracy for all sizes of angle.
+*
+*  Called:  slDVXV, slDVN, slDVDV
+*
+*  Last revision:   14 June 2005
+*
+*  Copyright P.T.Wallace.  All rights reserved.
+*
+*  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 V1(3),V2(3)
+
+      DOUBLE PRECISION V1XV2(3),WV(3),S,C
+      DOUBLE PRECISION slDVDV
+
+
+
+*  Modulus of cross product = sine multiplied by the two moduli.
+      CALL slDVXV(V1,V2,V1XV2)
+      CALL slDVN(V1XV2,WV,S)
+
+*  Dot product = cosine multiplied by the two moduli.
+      C = slDVDV(V1,V2)
+
+*  Angle between the vectors.
+      IF ( S.NE.0D0 .OR. C.NE.0D0 ) THEN
+         slDSEPV = ATAN2(S,C)
+      ELSE
+         slDSEPV = 0D0
+      END IF
+
+      END