for(a in As){
    for(b in Bs){
        for(c in Cs){
            for(d in Ds){
                
            }
        }
    }
}

data class A(
    val b: List<B>
)
data class B(
    val c: List<C>
)
data class C (
    val d: List<D>
        )
data class D(
    val string1: List<String>,
    val string2: String
)

.flatten()

List<String>

val deeplyNestedList: List<List<List<List<String>>>> = TODO()
val flatList: List<String> = deeplyNestedList
    .flatten()
    .flatten()
    .flatten()

for (x in deeplyNestedList.asSequence().flatten().flatten().flatten()) {
    println(x)
}

data class A(
    val b: List<B>
)
data class B(
    val c: List<C>
)
data class C (
    val d: List<D>
        )
data class D(
    val string1: List<String>,
    val string2: String
)

for ((string1, string2) in listOfA.asSequence().flatMap { it.b }.flatMap { it.c }.flatMap { it.d }) // ...

Type inference failed:

fun <T, R> Sequence<T>.flatMap
            (
    transform: (T) → Sequence<R>
)
: Sequence<R>
cannot be applied to

        receiver: Sequence<A>?
arguments:
(
        (A) → List<B>?
)

flatMap