code: function simulatedUserInput(numArray) {
      // pause + first one
      // forEach after the first: outer + inner
      const rest$ = numOrArray => {
        if (!numOrArray.length) return after(pause, () => numOrArray)
        let rest = after(pause, () => numOrArray[0])
        for (let i = 1; i < numOrArray.length; i++) {
          rest = concat(rest, after(outer, () => numOrArray[i]))
        }
        return rest
      }
  
      return from(numArray).pipe(concatMap(rest$))
    }
docs: an observable of numbers every second