code: func (f *SeriesFile) Retain() func() {
  	if f != nil {
  		f.refs.RLock()
  
  		// Return the RUnlock func as the release func to be called when done.
  		return f.refs.RUnlock
  	}
  	return nop
  }
docs: // Retain adds a reference count to the file.  It returns a release func.