inline fun <T, R> Sequence<T>.foldIndexed((source)
operation: (index: Int, acc: R, T) -> R
Accumulates value starting with initial value and applying operation from left to right
to current accumulator value and each element with its index in the original sequence.
Returns the specified initial value if the sequence is empty.
function that takes the index of an element, current accumulator value
and the element itself, and calculates the next accumulator value.
The operation is terminal.