code: func DropFolder(ll *Lowlevel, folder string) {
  	db := newInstance(ll)
  	db.dropFolder([]byte(folder))
  	db.dropMtimes([]byte(folder))
  	db.dropFolderMeta([]byte(folder))
  
  	// Also clean out the folder ID mapping.
  	db.folderIdx.Delete([]byte(folder))
  }
docs: // DropFolder clears out all information related to the given folder from the
  // database.