Subroutine:	IRPL_CORNERS.

 Purpose:      Calculate plate corners in plate-referenced coordinates.

 File:		irpl_standard.shl.

 Author: 	85 Jul 19 Uwe Peppel.
 
 Use: 
	call IRPL_CORNERS (
		size     I	doubleprecision(2),
		corner   O	doubleprecision(3,4).
		)
	size	long. and lat. length of a plate edge in radians 
		(= great circle angular distance),
	corner	xyz of the four corners of the plate in plate-
		referenced coordinates.
 Description: 
	The plate-referenced coordinate system is a rectangular
       system having the plate center at its x-axis. The y-axis
       points to the top of the plate.
       IRPL_CORNERS accepts any value for SIZE. In practice the
       maximum value for SIZE should be pi/2, which is a plate
       covering one half of the whole sky.
       The corners are counted anti-clockwise when the x-y-z-system
       is taken to be right handed and the plate is seen from outside
       the unit-sphere, beginning at the corner with y and z coordinates
       both having positive values. With this convention the top
       of the plate consists of the corners 1 and 4.

 Updates:
	900501 Sjag Steensma; documentation only.
	900926 DK, change plate definition