- A list copy holds each new copy as a root while the list is charged
for it, so a collection at that point no longer frees the copy. Before,
`x.setChildren(<c/> + <d/>)` near the heap limit could leave one child
twice or end with an internal error.
- An assignment to a path such as `x.a.b = v` or `x.a.b[0] = v` removes
the elements it added for the path when a later step stops at a limit
or throws.
- Comparing XML values counts the attribute tables and the work stack
against the heap limit.
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