Subroutine:   IRCO_PRECESS
 
 Purpose:      precess a coordinate system
 
 Category	WORLD COORDINATES, IRAS

 File:         irco.shl
 
 Author:       Do Kester
 
 Person resp.: Do Kester
 
 Address:      guspace!do or rugfx4!do         (uucp)
 
 Use: call irco_precess(
 	refcoor  	I  	integer 
 	year		I  	real
 	newcoor  	I/O 	integer )
 
 	refcoor	reference coordinate system
 	year	year to precess to
 	newcoor input: 0 => output: free coordinate number
		otherwise   output = input
 Description:  
 	Precession can be applied to the coordinate systems Equatorial
 	and Ecliptic (numbers 1 and 3). YEAR is the year to precess
	to e.g. 1950.0
	The name of the system will be adjusted to the new epoch.

 	If NEWCOOR equals REFCOOR all transformation matrices
 	that referred to REFCOOR will be precessed. If NEWCOOR
	is 0 upon entry, a free coordinate number will be returned,
	or 0 if none is available any more.
 	Upon output NEWCOOR contains the number of the precessed system. 

	Note that if NEWCOOR equals REFCOOR subsequent precessions
	will be with respect to the last result and more importantly
	errors will accumulate in the process.

	The connection to coordinate system nr 1 is always made, so that
	all connections with other systems will be present in one or
	two steps.

 Reference: Lieske etal. Astron & Astrophys. (1977) 58 p.1 - 16.

 Externals:    
 
 Updates:      5 jan 1990 DK, documentation only
		17 DEC 1990 DK, make always connection with system nr 1