Kotlin List permutations

permutations.kt

fun <T> List<T>.permutations(): Sequence<List<T>> {
    if (this.size == 1) return sequenceOf(this)
    val list = this
    return sequence {
        val sub = list.get(0)
        for (perm in list.drop(1).permutations())
            for (i in 0..perm.size) {
                val newPerm = perm.toMutableList()
                newPerm.add(i, sub)
                yield(newPerm)
            }
    }
}