inline fun <R> CharSequence.foldRightIndexed((source)
operation: (index: Int, Char, acc: R) -> R
Accumulates value starting with initial value and applying operation from right to left
to each character with its index in the original char sequence and current accumulator value.
Returns the specified initial value if the char sequence is empty.
operation - function that takes the index of a character, the character itself
and current accumulator value, and calculates the next accumulator value.