code: func (ev *evaluator) vectorSelectorSingle(it *storage.BufferedSeriesIterator, node *VectorSelector, ts int64) (int64, float64, bool) {
  	refTime := ts - durationMilliseconds(node.Offset)
  	var t int64
  	var v float64
  
  	ok := it.Seek(refTime)
  	if !ok {
  		if it.Err() != nil {
  			ev.error(it.Err())
  		}
  	}
  
  	if ok {
  		t, v = it.Values()
  	}
  
  	if !ok || t > refTime {
  		t, v, ok = it.PeekBack(1)
  		if !ok || t < refTime-durationMilliseconds(LookbackDelta) {
  			return 0, 0, false
  		}
  	}
  	if value.IsStaleNaN(v) {
  		return 0, 0, false
  	}
  	return t, v, true
  }
docs: // vectorSelectorSingle evaluates a instant vector for the iterator of one time series.