What does the header / impl keyword mean in kotlin?

Question

A few moments ago, when I was studying Kotlin github relay, I found something interesting: now Kotlin now has a header keyword.

CollectionsH.kt :

open header class ArrayList<E> : MutableList<E> { //... header inline fun <reified T> Array<out T>?.orEmpty(): Array<out T> 

What does it mean? Is this some kind of public function or an internal function to help develop stdlib?

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EDIT: There seems to be an impl keyword that works along with the header. What is it?

Answer 1

UPDATE 2:

Support for multi-platform project released.

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UPDATE 1:

The keywords impl and header were replaced by actual and expect in the preliminary release of multi-platform projects for Kotlin.

The official documentation can be found here or at the same time GitHub permalink

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This was discussed in this video at 14:28.

From slides :

In principle, header will be used when writing interfaces for modules operating on several platforms (JVM, JS and Native). The impl will be used when writing an implementation for a particular platform.