Sequence

Iterator

.iterator()

Iterable

asIterable()

asSequence

asSequence()

fun <T> Sequence<T>.defaultIfEmpty(value: T): Sequence<T> {
  val upstream = this;
  return object : Sequence<T> {
    private val delegate: Iterator<T> by lazy {
      val first = upstream.iterator()
      if (first.hasNext()) first else listOf(value).iterator()
    }
    override fun iterator() = object: Iterator<T> by delegate {}
  }
}

switchIfEmpty

listOf(value).iterator()

other.iterator()

switchIfEmpty(listOf(value))