This yields nodes as `Step`s. This allows keeping track of the structure of the tree as you walk through descendents, as each element will be yielded twice: both as a `Step::In` *and* as a `Step::Out`. Non-element nodes will be yielded once as a `Step::Around`.
I'm adding `walk` iteration specifically to avoid having to write certain methods recursively. Existing recursive callers of `children` should probably be updated to `walk` the tree and maintain a separate `Vec` stack.
David Wendt