code: func xFetch(now, expiry time.Time, delta time.Duration, beta float64, rf randFunc) bool {
  	offset := time.Duration(float64(delta) * beta * math.Log(rf()))
  	return !now.Add(-offset).Before(expiry)
  }
docs: // xFetch returns true if recomputation should be performed.
  //
  // The decision is probabilistic, based on the evaluation of a probability
  // distribution that scales according to the time since the value was last
  // recomputed.
  //
  // delta is a factor representing the amount of time that it takes to
  // recalculate the value. Higher delta values allow for earlier recomputation
  // for values that take longer to calculate.
  //
  // beta can be set to >1 to favor earlier recomputations; however, in practice
  // beta=1 works well.