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diff --git a/math/slalib/dvn.f b/math/slalib/dvn.f
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+      SUBROUTINE slDVN (V, UV, VM)
+*+
+*     - - - -
+*      D V N
+*     - - - -
+*
+*  Normalizes a 3-vector also giving the modulus (double precision)
+*
+*  Given:
+*     V       d(3)      vector
+*
+*  Returned:
+*     UV      d(3)      unit vector in direction of V
+*     VM      d         modulus of V
+*
+*  Notes:
+*
+*  1  If the modulus of V is zero, UV is set to zero as well.
+*
+*  2  To comply with the ANSI Fortran 77 standard, V and UV must be
+*     different arrays.  However, the routine is coded so as to work
+*     properly on most platforms even if this rule is violated.
+*
+*  Last revision:   22 July 2004
+*
+*  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 V(3),UV(3),VM
+
+      INTEGER I
+      DOUBLE PRECISION W1,W2
+
+
+*  Modulus.
+      W1 = 0D0
+      DO I=1,3
+         W2 = V(I)
+         W1 = W1+W2*W2
+      END DO
+      W1 = SQRT(W1)
+      VM = W1
+
+*  Normalize the vector.
+      IF (W1.LE.0D0) W1 = 1D0
+      DO I=1,3
+         UV(I) = V(I)/W1
+      END DO
+
+      END