class Generator[T] extends Iterable[T] {
  val buffer = scala.collection.mutable.ArrayBuffer[() => Iterable[T]]()

  protected def <<(t: => T) { buffer += (() => List(t)) }

  protected def <<<(t: => Iterable[T]) { buffer += (() => t) }

  override def iterator = buffer.toStream.flatMap(_.apply).iterator
}

foo

val g = new Generator[String] {
  this << "foo"
  this << { println("bar!"); "bar" }
  this <<< { println("baz!"); (1 to 3) map ("baz: "+_) }
}

g.iterator
g.iterator take 1 foreach (println _)