Implement an `EventEmitter` class with `on(event, callback)` to subscribe, `off(event, callback)` to unsubscribe a specific callback, and `emit(event, ...args)` to call every subscriber of that event in registration order, forwarding the arguments. Multiple callbacks may listen to the same event.
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The emitter keeps a map from event name to an ordered list of callbacks. `on` appends, `off` filters by reference, and `emit` iterates the list forwarding the arguments. This is the core publish/subscribe pattern behind Node's EventEmitter and browser event targets.
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