code: func (w *wrapper) SetFolder(fld FolderConfiguration) (Waiter, error) {
  	w.mut.Lock()
  	defer w.mut.Unlock()
  
  	newCfg := w.cfg.Copy()
  
  	for i := range newCfg.Folders {
  		if newCfg.Folders[i].ID == fld.ID {
  			newCfg.Folders[i] = fld
  			return w.replaceLocked(newCfg)
  		}
  	}
  
  	newCfg.Folders = append(newCfg.Folders, fld)
  
  	return w.replaceLocked(newCfg)
  }
docs: // SetFolder adds a new folder to the configuration, or overwrites an existing
  // folder with the same ID.