The F1 integration test mocks the open-set as {} so openIds is always empty — every
node hits the collapsed branch, and the open-keep + recursion path (keep an OPEN
branch's children, recurse to prune a nested collapsed child) runs in zero tests. Add
a unit test: open parent (kept with children) → nested collapsed child (pruned to []),
plus a top-level collapsed node (pruned), with hasChildren preserved and immutability
asserted. Non-vacuous: clearing an open branch fails (a); removing recursion fails (b).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
agent_coder
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