In retrospect, it's not much good for transforming the structure, but great for many other things.
For everything else, the other tree functions like trav, treewise, ontree, etc. might be more useful.
How would you make a more general purpose version of maptree that could encompass your transform function? It needs to be able to selectively modify some branches of the tree, and avoid calling map on them. Mine can't do that because it restricts you to atoms. Maybe if it took in two functions, one that operates on atoms, and one on lists, which can then optionally continue the mapping on a subnode. But then what has been gained? You're pretty much writing the whole pattern over again anyway.
Hmm. There seems to be a pattern here, but I can't see how to abstract it out, beyond the somewhat useful but restricted maptree posted above. Ideas?
