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      DOUBLE PRECISION FUNCTION slDSEP (A1, B1, A2, B2)

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      D S E P
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  Angle between two points on a sphere (double precision)

  Given:
     A1,B1    dp    spherical coordinates of one point
     A2,B2    dp    spherical coordinates of the other point

  (The spherical coordinates are RA,Dec, Long,Lat etc, in radians.)

  The result is the angle, in radians, between the two points.  It
  is always positive.

  Called:  slDS2C

  P.T.Wallace   Starlink   April 1985

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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