Avoid users of recommended approach having to set arrayenv key to mak…

[cfr42]

…e it work, without overriding any explicit user setting. Use format's key-val processing to avoid kvoptions and process keys only once in fitchproof(*). Add test/demo file to check this works as claimed.

[rzach]

This is lovely, and thank you, but there is a problem: anyone who uses \setkeys{fitch} in an existing document will now get errors. I don't see an easy solution, do you?

[cfr42]

Well, there is a fairly easy solution, but I do not like it much. I've pinged David Carlisle from the tagging-project discussion in case I'm missing something re. keyval.sty. The long way would be to set the keys up there as well as an interim measure, but that is very clunky.

If keyval isn't loaded at all, it's fine, of course, but when it is, there doesn't seem a nice way of setting just an unknown key handler. So it works OK with a wrapper command, but that is quite impolite to keyval :(.

I tried to use a hook, but I'm sure it is too late ...

[rzach]

When I break things in v2.0 I can break this too. But I don't want to break things until I have to.