Direct product in Kotlin

direct-product.kt

fun <E> Collection<Collection<E>>.directProduct(): List<List<E>> =
    if (isEmpty()) {
        listOf()
    } else {
        val (head, rest) = first() to drop(1)
        head.map(::listOf).let {
            if (rest.isEmpty()) {
                it
            } else {
                rest.directProduct().flatMap { tail -> it.map { h -> h + tail } }
            }
        }
    }