> For example, if you have a tree and you want to change one leaf on the tree, the only thing you really need to copy is the part of the tree that's actually different now - for the unchanged branches, the new tree just has a pointer to branches of the old tree, so they can be (and are) reused without copying.
This still only works if a child has one / a known-ahead-of-time number of parents. If you need to update an object that N objects point to, you need to update all N references.
