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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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