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func combine(queries []Query) Sum {
  var sum Sum

  var wg sync.WaitGroup
  sumsChan := make(chan Sum)

  // Limit to 10 concurrent requests
  limiter := make(chan struct{}, 10)

  // Fetch sum for each query
  for _, query := range queries {
    wg.Add(1)
    go func(query Query) {
      // Rate limiting
      limiter <- struct{}{}

      // Push to the sums channel
      sumsChan <- fetch(query)

      wg.Done()
      <-limiter
    }(sum)
  }

  // Add the sums as they are fetched
  go func() {
    for sumEl := range sumsChan {
      sum = add(sum, sumEl)
    }
  }()

  // Wait for all of the wg to be done being fetched
  wg.Wait()
  close(sum)
  close(limiter)

  return sum
}