Class Blackboard

Number of stored keys.

Read key, typed by the caller's generic (defaults to any). undefined if unset.

Read key, returning fallback (and inferring its type) when the key is unset.

Write value at key, replacing any prior value.

Whether key currently holds a value.

Remove key; returns whether it was present.

Drop every entry (e.g. on a match restart).

The live set of keys (iteration order = insertion order).