What's a good data structure for building equivalence classes on nodes of a tree?

You seem to have two forms of equivalence to deal with. Plain equivalence (A), tracked as equivalence classes which are kept up to date and structural equivalence (D), for which you occasionally go build a single equivalence class and then throw it away.

I don't think any one structure is going to solve your problems, but you might take a look at the Disjoint-set data structure. An equivalence class, after all, is the same thing as a partitioning of a set. It should be able to handle some of those operations speedily.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.