code: def move(self, key, folder):
          
          # Basically this is a sophisticated __delitem__
          # We need the path so we can make it in the new folder
          path, host, flags = self._exists(key)
          self._invalidate_cache()
  
          # Now, move the message file to the new folder
          newpath = joinpath(
              folder.base, 
              folder.get_name(), 
              "cur",     # we should probably move it to new if it's in new
              basename(path)
              )
          self.filesystem.rename(path, newpath)
          # And update the caches in the new folder
          folder._invalidate_cache()
docs: Move the specified key to folder.
  
          folder must be an MdFolder instance. MdFolders can be obtained
          through the 'folders' method call.