@SašoŽivanović Maybe I just don't understand how to use it, but wouldn't it be much more useful if it used explicitly specified arguments? As I understand it, The false/true codes are essentially of type _. I can't make them type O say. So I can't use it to say 'if such-and-such, use option so-and-so; otherwise, use some other option', for example. Have I misunderstood something? I guess I hoped it would reduce the need to repeat code.

          if is auto iteration={
            if iteration=1{
              edge label/.process={Ow}{iteration}{node [midway, anchor=south east, left, xshift=-2.5pt, font=\scriptsize] {$\frac{2}{n}$}},
            }{
              edge label/.process={Ow+nw}{iteration}{#1-1}{node [midway, anchor=south east, xshift=-2.5pt, left, font=\scriptsize] {$\frac{2}{n-#1}$}},
              if phantom={}{
                !u.edge label/.process={OW+nw2}{iteration}{#1-2}{node [midway, below, font=\scriptsize] {$\frac{n-#1}{n-#2}$}},

If I could do some of the testing in the .process, then I could avoid some of the repetition e.g. by using \frac{2}{#1}, where #1 depended on whether iteration was 1 etc. I realise I couldn't nest ifs that way, of course, but it would reduce the amount of repetition. However, I can't work out how to do this. I tried to use a P to do some of it, but got thoroughly confused and gave up :(.

@SašoŽivanović Also, I can't figure out how to make the edge angles constant in this tree:

I don't know if I could do something like

I thought of wrapping the result of the ?, but the trouble is that I want +n if a condition holds, but not otherwise, and that doesn't seem possible because only one result ends up in the queue and I can't process that result in any condition-sensitive way.

@SašoŽivanović But I suspect I'm just not understanding how I'm meant to use this.