@cfr A trick I have recently discovered to output a comma-separated toks/keylist using an arbitrary delimiter (here ;): split={...}{,}{TeX,{TeX={; },TeX}} ;-)
@SašoŽivanović However, I think I'd need something like this
split option={proof tree rhestr rhifau llinellau cau}{{content+={\ },content+},{content+={,\,},content+}},
which seems horrendous. I think the current use of toks to fake a keylist effect is simpler ;).
(And it will be worse for the justifications, probably as in one variant, I'd need to reverse the list first.)
I really just wanted to make sure I wasn't missing something obvious as this seems quite a common feature of keylists in LaTeX. But I think your keylists are not really designed for the same purpose.
