def get_best_mutable_version():
    """Return a Deferred that fires with an IMutableFileVersion for
        the 'best' available version of the file. The best version is
        the recoverable version with the highest sequence number. If no
        uncoordinated writes have occurred, and if enough shares are
        available, then this will be the most recent version that has
        been uploaded.

        If no version is recoverable, the Deferred will errback with an
        UnrecoverableFileError.
        """
