The confusing part may be the implicit bind that happens
when you do

	bind -ca /sys/src/cmd $home/kern/cmd

The resulting mount point has to things in the link
list bound there:

	/sys/src/cmd	- containing the create flag
	$home/kern/cmd	- not containing the create flag

The latter is implicit since you were binding -a onto a
directory.  You would have gotten the semantics you
wanted if you had said instead:

	bind -c $home/kern/cmd $home/kern/cmd
	bind -ca /sys/src/cmd $home/kern/cmd