def dequeue(filebase):
    """Return the message and metadata contained in the named file.

        filebase is the base name of the message file as returned by the
        .enqueue() method.  This file must exist and contain a message and
        metadata.  The message file is preserved in a backup file, which must
        be removed by calling the .finish() method.

        Returned is a 2-tuple of the form (message, metadata).
        """
