code: func (f *Field) Value(columnID uint64) (value int64, exists bool, err error) {
  	bsig := f.bsiGroup(f.name)
  	if bsig == nil {
  		return 0, false, ErrBSIGroupNotFound
  	}
  
  	// Fetch target view.
  	view := f.view(viewBSIGroupPrefix + f.name)
  	if view == nil {
  		return 0, false, nil
  	}
  
  	v, exists, err := view.value(columnID, bsig.BitDepth())
  	if err != nil {
  		return 0, false, err
  	} else if !exists {
  		return 0, false, nil
  	}
  	return int64(v) + bsig.Min, true, nil
  }
docs: // Value reads a field value for a column.