@SašoŽivanović Yes, but read the comments. crocket wants a solution for branching trees as well. The MWE in that question is poorly chosen.

@SašoŽivanović Is that better than saying node options/.register=<whatever>. Or are you saying that process keylist register would be the thing to use with declare keylist register?

@SašoŽivanović Interesting. Thanks. You'd still have the problem of splitting it, of course, and version 2 doesn't help in this case because crocket is still using version 1.

@SašoŽivanović But you can't just use that within a tree, right? I just wanted to apply a list of TikZ keys, say, to a given node.

But you can't say something like

\documentclass[tikz,border=10pt,multi]{standalone}
\usepackage{forest}
\begin{document}
\forestset{%
  declare keylist register={mykeylist},
  mykeylist={text=red},
}
\begin{forest}
  [a[b][c, process keylist register=mykeylist]]
\end{forest}
\end{document}

which is obviously much too naïve!

@SašoŽivanović Can I use the max in nodewalk to get the maximum level for a tree? At least theoretically?

to line doesn't really work unless a node has no children. At least, I can't figure out how to adjust things properly. I should really move it somewhere, but then everything goes wrong. Right now, I give up and I'm just going to say that if you use to line for a node, you have to use it for every descendant of the node. I've managed to stop it automatically adding unwanted edges, I think, so maybe that will have to do.

Or maybe I should just not let anybody else play with this. It is bound to go horribly wrong.

@cfr My bad late at night):You want to use node options/.register=..., yes, as this will add the contents of the register to the options passed to tikz's\node. process keylist register processes the keylist immediately with forest.

@cfr I'll post an update to crocket's question soon, with a branching tree. But I believe he'll have to switch to v2. The only was to do partial outputs in v1 is by making nodes invisible, and then I wouldn't know how to do the alignment.

@cfr Actually your "naive" example should work. The reason it doesn't is due to a bug: registers, as opposed to nodes, are not initialized. The error occurs because mykeylist={text=red} adds to a non-initialized register. The example works fine with mykeylist'={text=red}. I'll fix this. Thanks.

@cfr I'm afraid I don't know enough about to line to help ...