def set_nodes(entries, overwrite=True):
    """Add multiple children to a directory node. Takes a dict mapping
        unicode childname to (child_node, metdata) tuples. If metdata=None,
        the original metadata is left unmodified. Returns a Deferred that
        fires (with this dirnode) when the operation finishes. This is
        equivalent to calling set_node() multiple times, but is much more
        efficient."""
