Check if we have a computation, not just an override.
Avoid setting -1 flags on the root node.
Add some asserts to check_cleared().
Users should never be modifiables.
Reclaim some computations immediately.
Reclaim some more computations immediately.
Don't find callers when their results are cleared.
Update version for regs as well so that weak refs could work.
Check that the version goes up when a reg is freed.
Remove redundant test.