Пытаюсь эту фигню чисто функционально сделать /* * This function moves

Question

Пытаюсь эту фигню чисто функционально сделать /* * This function moves

all the non-null elements to the beginning of the list (by removing nulls) and merges equal elements.
* The parameter 'double' specifies the way how to merge equal elements:
* it returns a new element that should be present in the result list instead of two merged elements.
*
* If the function double("a") returns "aa",
* then the function moveAndMergeEqual transforms the input in the following way:
* a, a, b -> aa, b
* a, null -> a
* b, null, a, a -> b, aa
* a, a, null, a -> aa, a
* a, null, a, a -> aa, a
*/
fun <T : Any> List<T?>.moveAndMergeEqual(double: (T) -> T): List<T> =