code: func (v *view) sum(filter *Row, bitDepth uint) (sum, count uint64, err error) {
  	for _, f := range v.allFragments() {
  		fsum, fcount, err := f.sum(filter, bitDepth)
  		if err != nil {
  			return sum, count, err
  		}
  		sum += fsum
  		count += fcount
  	}
  	return sum, count, nil
  }
docs: // sum returns the sum & count of a field.