kotlin collections build-list

kotlin.collections.build-list.kt

For those cases where you have to calculate the elements before adding them to the list, 
you "save" the boilerplate of creating a mutable list and then returning an immutable list.
The example of the docs is... but it is understood where it goes.

val y = buildList () {
         add ('a')
         addAll (x)
         add ('d')
         }
         
 https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/build-list.html