@cfr I'm back. I just checked out prooftrees on ctan. Wow. The documentation is fantastic. Wow. ---Jelaously wishing I had your gift for writing it ... ;-)

@cfr Indeed (for both statements). This would make sense only if you needed to typeset a keylist that you need in pgfkeys keylist format for some other reason in the first place.

@cfr Reading your comment at answer tex.stackexchange.com/a/300597/16819, I realized that if one wanted to do multiple page breaks, filter would come handy to typeset the middle part(s): for nodewalk={draw tree processing order/.style={filter={level<=3}{level()>1}}}{},

@SašoŽivanović Interesting. I think it is probably less about page breaks and more about inserting explanation. Will have to think about it.

@SašoŽivanović Do you have any idea why I get errors if I use a macro defined using \ensuremath in maths mode in a justification? I don't get this with the version on CTAN which uses 2.0.2 of Forest, but I do with the code I've tweaked a bit to use the newer version you sent me. [I'm not sure if it is because of the tweaks or not.]

@cfr Yes, inserting the explanation is certainly the hard bit.

@cfr Hmm, I can't reproduce the error. Can you send me the problematic file?

@SašoŽivanović I've emailed you an example with the problematic version of prooftrees.sty, which needs the forest.sty you sent me for the filter stuff.

@SašoŽivanović Thanks for agreeing to take a look. Don't worry if it takes too much time, obviously.

@cfr No problem. The crazy week is over ;-)

@cfr While I'm not sure about the details, like why the error occurs only when \ensuremath is embedded within $...$. this is almost certainly an expansion issue. Defining \lif using \noexpand\ensuremath is a workaround, so I guess that \ensuremath in just option gets expanded somewhere.

@cfr Predictably, it was me who fucked up. I was "fixing" the definitions of split option/register. Fixed the fix now, and sent it over email (2.0.3rc3). Thanks for finding the zillionth bug.

@SašoŽivanović Thanks for investigating. I tried comparing the two versions of forest.sty, but there were many differences and I didn't know what to look for or if something I'd done had broken it. Thanks for fixing it.

@SašoŽivanović ;)