docs: def _get_message(self, key, since=None):
          """Return the MdMessage object for the key.
  
          The object is either returned from the cache in the store or
          made, cached and then returned.
  
          If 'since' is passed in the modification time of the file is
          checked and the message is only returned if the mtime is since
          the specified time. 
  
          If the 'since' check fails, None is returned.
  
          'since' must be seconds since epoch.
          """
          stored = self.store[key]
          if isinstance(stored, dict):
              filename = stored["path"]
              folder = stored["folder"]
              if since and since > 0.0:
                  st = stat(filename)
                  if st.st_mtime < since:
                      return None
              stored = MdMessage(
                  key, 
                  filename = filename, 
                  folder = folder,
                  filesystem = folder.filesystem
                  )
              self.store[key] = stored
          else:
              if since and since > 0.0:
                  st = stat(stored.filename)
                  if st.st_mtime < since:
                      return None
                  
          return stored