Annotations We Might Want in the Future
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    - target-invalid
      the target might have changed (registers, heap contents, or
      execution status).  For performance, we might eventually want
      to hit `registers-invalid' and `all-registers-invalid' with
      greater precision

    - systematic annotation for set/show parameters (including
      invalidation notices).

    - similarly, `info' returns a list of candidates for invalidation
      notices.