Previously, the intention of upper case -Z was the match -W/--width and
-H/--height, which are uppercase to avoid conflicts with -h/--help.
But -z/--depth isn't _really_ related to -W/-H.
This avoids a conflict with -Z/--lebesgue, but may conflict with
-z/--cat. Fortunately we don't currently have any conflicts with the
latter. Since -z/--depth and -Z/--lebesgue are both disk-layout related,
the risk of conflicts are probably much higher there.
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