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      REAL FUNCTION slBEAR (A1, B1, A2, B2)

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      B E A R
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  Bearing (position angle) of one point on a sphere relative to another
  (single precision)

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

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

  The result is the bearing (position angle), in radians, of point
  A2,B2 as seen from point A1,B1.  It is in the range +/- pi.  If
  A2,B2 is due east of A1,B1 the bearing is +pi/2.  Zero is returned
  if the two points are coincident.

  P.T.Wallace   Starlink   23 March 1991

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

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