code: func coalesceBuckets(buckets buckets) buckets {
  	last := buckets[0]
  	i := 0
  	for _, b := range buckets[1:] {
  		if b.upperBound == last.upperBound {
  			last.count += b.count
  		} else {
  			buckets[i] = last
  			last = b
  			i++
  		}
  	}
  	buckets[i] = last
  	return buckets[:i+1]
  }
docs: // coalesceBuckets merges buckets with the same upper bound.
  //
  // The input buckets must be sorted.