如何理解python中的嵌套生成器?

我有这个密码:

def flat_on_level1(it, d=-1, level=None):
    """

    >>> list(flat_on_level1([[[['a']]]], level=3))
    [['a']]

    """
    if d==-1:
        return list(flat_on_level1(it, d=d+1, level=level))

    if d==level:
        return (i  for i in [it])

    for x in it:
        yield from flat_on_level1(x, d=d+1, level=level)

def flat_on_level(it, d=-1, level=None):
    """
    >>> list(flat_on_level([[[['a']]]], level=3))
    [['a']]

    """
    if d == -1:
        return list(flatt_on_level(it, d=d + 1, level=level))

    if d == level:
        return (i for i in [it])

    res = []
    for x in it:
        res.extend( flat_on_level(x, d=d+1, level=level))
    return res

我得到一张空名单。这是怎么回事?