| An Infrastructure-opaque type used for tagging various blackboard objects
with Plugin-specific information. The infrastructure will never copy
or transmit Annotations between agents, but will save them when
persisting (hence the extends Serializable).
Annotations may be attached to appropriate plan objects at object
construction time. They should never be modified (or even examined)
by any entity other than the one that created the plan object.
This structure is intentionally opaque to the infrastructure.
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