def parents(self, ix):
    if (ix is None):
        return []
    if (ix <= self._walkfrom):
        self._walkto(ix)
    return self._children.get(ix, [])
def parents(self, ix):
    if (ix is None):
        return []
    if (ix <= self._walkfrom):
        self._walkto(ix)
    return self._children.get(ix, [])