def get_writekey():
    """Return this filenode's writekey, or None if the node does not have
        write-capability. This may be used to assist with data structures
        that need to make certain data available only to writers, such as the
        read-write child caps in dirnodes. The recommended process is to have
        reader-visible data be submitted to the filenode in the clear (where
        it will be encrypted by the filenode using the readkey), but encrypt
        writer-visible data using this writekey.
        """
