Task: implement a class with subscribe, unsubscribe, and next methods (Observer pattern). All methods should be strictly typed in TypeScript. The subscribe method should return an unsubscribe function.

typescript class Observer<T> { private observers: Set<(data: T) => void> = new Set();

subscribe(fn: (data: T) => void): () => void { this.observers.add(fn); // Return unsubscribe function return () => this.unsubscribe(fn); }

unsubscribe(fn: (data: T) => void): void { this.observers.delete(fn); }

next(data: T): void { this.observers.forEach(fn => fn(data)); } }

// Example usage: const obs = new Observer<number>(); const unsubscribe = obs.subscribe(value => console.log('Received:', value)); obs.next(42); // Will output: Received: 42 unsubscribe(); obs.next(100); // Will not output anything