// util[0].Time and end.
	muInWindow := func(util []trace.MutatorUtil, end int64) float64 {
		total := 0.0
		var prevU trace.MutatorUtil
		for _, u := range util {
			if u.Time > end {
				total += prevU.Util * float64(end-prevU.Time)
				break
			}
			total += prevU.Util * float64(u.Time-prevU.Time)
			prevU = u
		}
		return total / float64(end-util[0].Time)
	}
	update := func() {
		for i, u := range util {