This call un-links an association that was made using bind_top_down.
    Have a look at help(bind_top_down)

def split_top_down(lower, upper, __fval=None, **fval):
    """This call un-links an association that was made using bind_top_down.
    Have a look at help(bind_top_down)
    """
    if __fval is not None:
        fval.update(__fval)
    if lower in upper._overload_fields:
        ofval = upper._overload_fields[lower]
        if any(k not in ofval or ofval[k] != v for k, v in six.iteritems(fval)):  # noqa: E501
            return
        upper._overload_fields = upper._overload_fields.copy()
        del(upper._overload_fields[lower])