KotlinFlowIntegerStream.kt

        val toIntegerStream = { string: String ->
            IterableSpliterator.of(string.chars().boxed().spliterator()).asFlow()
        }

        val histoOfLetters = { word: String ->
            flow {
                emit(toIntegerStream(word).fold(HashMap<Int, LongWrapper>()) { accumulator, value ->
                    var newValue: LongWrapper? = accumulator[value]
                    if (newValue == null) {
                        newValue = LongWrapper.zero()
                    }
                    accumulator[value] = newValue.incAndSet()
                    accumulator
                })
            }
        }