code: func (h *Histogram) ApproxSum() float64 {
  	if h.useLocks {
  		h.mutex.RLock()
  		defer h.mutex.RUnlock()
  	}
  	sum := 0.0
  	for i := uint16(0); i < h.used; i++ {
  		midpoint := h.bvs[i].midpoint()
  		cardinality := float64(h.bvs[i].count)
  		sum += midpoint * cardinality
  	}
  	return sum
  }
docs: // Approximate sum