Investigate doing the iterators using iterator composition

Write the iterators as a recursive tree traversal that returns iterators.

Now that lots of stuff is wrapped in an arc, the stack usage should be minimal, so you should be able to traverse really deep trees with a small stack. Something like a few 1000 deep would seem to be a good limit.

Note: this needs precise understanding of the stack usage, including tests, to make sure we can traverse large trees.

Locate the current iterator implementations and traversal tests first, then measure stack usage on trees a few thousand levels deep. Compare the existing traversal with iterator composition and document whether the proposed approach meets the stated depth and stack-use target; done means the implementation and tests demonstrate safe traversal at that limit.

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