def free(me, n=None):
    """set it all free if n is None.
otherwise delete only the n-th bound."""
    assert me.up.id
    me.listen(False)
    if n is None:
        me.up.blist = []
    else:
        del me.up.blist[n]
    me.update(False)
    me.listen(True)
    return me
