Code changes look solid. My only comment is that it's not entirely obvious what the benefit of this feature is. I think you and I discussed being able to include loop-like structures in the tree itself. Would you be able to do that with this functionality? Is the intention of the callback to dynamically build and/or modify the tree such that it creates a loop?

You could call other trees from this but it wouldn't be able to pass information back to the origin i dont think. Uma wanted something like this to be able to build an output dataset in-situ while the tree ran. I think it's good to have callback triggers for the tree anyway. increase the options for dynamic interaction.