This enum is used by dataflow analyses to indicate whether further
propagation is necessary to reach the fix point. Accidentally discarding
such a value will likely lead to propagation stopping early, leading to
incomplete or incorrect results. The most egregious example is the
duality between `join` on the analysis class, which triggers propagation
internally, and `join` on the lattice class that does not and expects
the caller to trigger it depending on the returned `ChangeResult`.
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