In our previous attempt to optimize this problem, we did not track the type of the
visit to the node, e.g. whether it came from evaluating Requires or Requires.private,
which resulted in some solutions being correctly incalculated due to greedy optimization
of the dependency graph.
We reintroduce this optimization by adding a second traversal serial as well as
re-introducing the PROPF_VISITED node property as well as a new PROPF_VISITED_PRIVATE
node property flag. This allows a node to be revisted at maximum two times per
traversal level.
Co-authored-by: Yi Chou <yich@google.com>
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