Possible Combinations
	USMARC	UTILITY	EXPLANATION
4xx	nnnn	BLANK	Simple cross reference
			Do not supply any }w values
	nnan	nna	Linking reference to a pre-AACR2
			heading (optional)
	nnaa	nnaa	Pre-AACR2 heading is not a valid
			AACR2 reference
	nnen	nne	Valid cross reference was also an
			earlier AACR2 heading
	nnnb	nnnb	Reference not displayed; 664
			field used (may find but never use)
5xx	nnnn	BLANK	Simple see also reference
			Do not supply any |w values
	annn	a	Earlier heading
	bnnn	b	Later heading
	nnnc	nnnc	See also reference for
			pseudonyms (663 field used)
	nnna	nnna	Reference not made (displayed)
	bnna	bnna	cf. RI26.3B-C(1b), p.2
	anna	anna	(unlikely ever to use)