`bind -c 9 9' remains useful, however, to retain createability of the
directory in which a union mount is then performed:

; cd /tmp
; mkdir x
; bind -a /sys/lib x
; ns|tail -4
mount -b '#s/rio.geoff.73279' /dev
bind  /tmp/x /tmp/x
bind -a /sys/lib /tmp/x
cd /tmp
; >x/FOO
x/FOO: rc: can't open: mounted directory forbids creation

versus

; unmount x
; bind -c x x
; bind -a /sys/lib x
; ns|tail -4
mount -b '#s/rio.geoff.73279' /dev
bind -c /tmp/x /tmp/x
bind -a /sys/lib /tmp/x
cd /tmp
; >x/FOO
; unmount x; ls x
x/FOO