Russ Cox reported ~0ul should be TWID32.
Reference: /n/sources/patch/applied/venti-twid32
Date: Fri May 13 23:24:09 CES 2011
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/venti/srv/dat.h	Fri May 13 23:23:37 2011
+++ /sys/src/cmd/venti/srv/dat.h	Fri May 13 23:23:30 2011
@@ -48,7 +48,7 @@
 	MaxIo			= 64*1024,	/* max size of a single read or write operation */
 	ICacheBits		= 16,		/* default bits for indexing icache */
 	MaxAMap			= 31*1024,	/* max. allowed arenas in an address mapping; must be < 32*1024 */
-	Unspecified		= ~0ul,
+	Unspecified		= TWID32,
 
 	/*
 	 * return codes from syncarena