flatten

fun <T> Iterable<Iterable<T>>.flatten() = object: Iterable<T> {
    val collection = this@flatten.Ifilter { it.isEmpty() }.iterator()
    var element = collection.nextOrNull()?.iterator()
    override fun iterator() = iterate {
        if (element.hasNext())
            return@iterate element!!.next()
        else if (collection.hasNext()) {
            element = collection.next().iterator()
            return@iterate element!!.next()
        }
        else
            return@iterate null
    }
}