+case $zoom in
+ */*/*) ;;
+ *[!0-9/]*) ;;
+ */*) ok=t mul=${zoom%/*} div=${zoom#*/} ;;
+ *) ok=t mul=1 div=$zoom ;;
+esac
+case $ok in nil) echo >&2 "$0: bad zoom $zoom"; exit 2 ;; esac
+case $(( $mul > $div )) in 1) echo >&2 "$0: zoom too high"; exit 2 ;; esac
+
+ok=nil