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My tentative conclusion is that we can eliminate submodule and subtree from consideration; subrepo should be investigated further. In particular we should see how it does with a three-level hierarchy.
See also this discussion of reorganization and beginning experiments with subrepro.
Scripting bridge
Each subpackage or subsubpackage (that is, anything with an SConscript in its top directory) goes in a separate repository, as with previous strategy, but we write our own custom tool to help with the scenarios we expect to encounter, and not much more.
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