inline fun <R : Any, C : MutableCollection<in R>> CharSequence.mapIndexedNotNullTo(
transform: (index: Int, Char) -> R?
Applies the given transform function to each character and its index in the original char sequence and appends only the non-null results to the given destination.
transform - function that takes the index of a character and the character itself
and returns the result of the transform applied to the character.