when {
        this === NONE -> return other
        this is AggregateEventListener -> this.eventListeners
        else -> arrayOf(this)
      }
    val right =
      when {
        other === NONE -> return this
        other is AggregateEventListener -> other.eventListeners
        else -> arrayOf(other)
      }

    return AggregateEventListener(left + right)
  }

  fun interface Factory {
    /**