The Jamfile here has tests for issues relating to this bug report, from
the jamming mailing list:

	[jamming] J= bug?
	Date: Tue, 12 Oct 2004 18:43:26 -0700
	From: Paul Forgey <paulf@metainfo.com>
	To: Jamming mailing list (jamming@perforce.com) <jamming@perforce.com>
	Is this a bug or am I missing something?  The following Jamfile 
	illustrates my question clearly enough.
	
	VAR = 1 2 3 4 ;
	Echo $(VAR:J=.) ;
	Echo $(VAR[2-]:J=.) ;
	# the next line seems to evaluate as empty
	Echo $(VAR[2-3]:J=.) ;
	# ..but you can see $(VAR[2-3]) isn't empty
	Echo $(VAR[2-3]) ;


The Jamfile here checks several other possible problems as well.
It's a bit crude - you'll have to read the output carefully.