code: func (h *Histogram) ValueAtQuantile(q float64) float64 {
  	if h.useLocks {
  		h.mutex.RLock()
  		defer h.mutex.RUnlock()
  	}
  	q_in := make([]float64, 1)
  	q_in[0] = q
  	q_out, err := h.ApproxQuantile(q_in)
  	if err == nil && len(q_out) == 1 {
  		return q_out[0]
  	}
  	return math.NaN()
  }
docs: // ValueAtQuantile returns the recorded value at the given quantile (0..1).