> I have a strong preference for teating whether functionality is
> available, rather than testing version numbers or strings.

Agreed.

> Not all shells that support "set -o tabcomplete" will set
> $KSH_VERSION, whereas code that says "if 'set -o tabcomplete' works
> then do 'set -o tabcomplete'" will always do the right thing.

Yes...until someone implements a "set -o tabcomplete" that does
something other than what you expect.  (In the case of tabcomplete,
that's relatively unlikely; this is really a more general comment.)  In
some cases, you can test whether the syntax you're interested in
actually does what you expect, but for a user-interface thing like tab
completion, that's approximately impossible.

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