fun main() = runBlocking {
  	val numbersFlow = flowOf(1,2,3).onEach { delay(1000) }
    val lettersFlow = flowOf("A", "B","C").onEach { delay(2000) }

    numbersFlow.combine(lettersFlow) { number, letter ->
     "$number - $letter"
    }.collect {
        println(it)
    }
}

// 1 - A
// 2 - A
// 3 - A
// 3 - B
// 3 - C

numbersFlow.onStart { emit(null) }.combine(
    lettersFlow.onStart { emit(null) }
) { number, letter ->
    ...
}