flatMapIndexedTo
Platform and version requirements: JVM (1.4), JS (1.4), Native (1.4) 
@JvmName("flatMapIndexedIterableTo") inline fun <T, R, C : MutableCollection<in R>> Sequence<T>.flatMapIndexedTo(
    destination: C, 
    transform: (index: Int, T) -> Iterable<R>
): C @JvmName("flatMapIndexedSequenceTo") inline fun <T, R, C : MutableCollection<in R>> Sequence<T>.flatMapIndexedTo(
    destination: C, 
    transform: (index: Int, T) -> Sequence<R>
): C Appends all elements yielded from results of transform function being invoked on each element and its index in the original sequence, to the given destination.
The operation is terminal.
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Licensed under the Apache License, Version 2.0.
    https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/flat-map-indexed-to.html