old_contents: export default function stereo () {
    }
  
new_contents: let emitterProto = {
    on(event, listener) {},
    once(event, listener) {},
    off(event, listener) {},
    emit(event, listener) {}
  };
  
  export default function stereo () {
    return Object.create(emitterProto);
  }
  
subject: Define the prototype