NAME:
	lhr_deruiter
 PURPOSE:
       This routine calculates the likelyhood ratio for a set
	of identifications according to De Ruiter et Al. 1977.
	It returns a 3-elements vector containing identification fraction 
	(used as an empirical p(id), completeness and reliability for the 
	entire set
 CALLING SEQUENCE:
	result=lhr_deruiter(xmmdb,wfcdb[,dmax,radec_err=radec_err,$
	radec_fromdb=radec_fromdb,lhr=lhr,rel=rel])
 INPUTS:
	xmmdb: xmm database 
	wfcdb: wfc dbatabase for extracting magnitudes of candidates
 OPTIONAL INPUT PARAMETERS:
	dmax: max. offset for nearest matches
	radec_err: no (an error to be assigned to every xmm position)
	radec_fromdb: if set, position error extracted for each object from
		xmmdb
 OPTIONAL OUTPUT PARAMETERS:
	lhr: if set to a name variable, gets filled with 2d array of lhr values
		for each candidate for each source
 RESTRICTIONS:
	UNTESTED
 CALLED PROCEDURES:
	
 MODIFICATION HISTORY:
	Original by Ignas Snellen
	08-Dec-2000  S. Aigrain: adapted as a separate function
	16-Jan-2001  S. Aigrain: de ruiter method separately

(See /home/xmmssc/soft/idl/pro/lhr/lhr_deruiter.pro)