`fail2ban-client` uses nested subcommand syntax and intermixes fixed/enumerable
values with dynamically detected ones. If you know exactly what your overall
command structure looks like, these completions will work great. Unfortunately
their discoverability is a bit lacking, but that's not really fish's fault.
e.g.
* `f2b-c get/set` take certain known values but also accepts a dynamic jail name
* `f2b-c get/set <jail>` take certain fixed options but...
* `f2b-c get/set <jail> action` require enumerating an entirely different set
of values to generate the list of completions, bringing us to...
* `f2b-c get <jail> action <action>` has a fixed number of options but
* `f2b-c set <jail> action <action> <property>` can be any valid command and its
arguments
The intermixing of fixed, enumerable, and free-form inputs in a single command
line is enough to make one's head spin!
