XPath gets lost when indexing children

[anonymous]

[Sorry if my terminology is inaccurate, but I am no XML expert...]

Here is what I do:

from genshi.input import html
from genshi.filters import Transformer
html = HTML('<ul><li>item1<ul><li>subitem11</li></ul></li><li>item2<ul><li>subitem21</li></ul></li></ul>')
print html | Transformer('li[1]/ul').attr('attribute', 'value')

Then I do get what I expect (sub-ul of first top-level li selected):

<ul><li>item1<ul attribute="value"><li>subitem11</li></ul></li><li>item2<ul><li>subitem21</li></ul></li></ul>

But if:

print html | Transformer('li[2]/ul').attr('attribute', 'value')

I would expect that sub-ul of second top-level li be selected), but here is what I get:

<ul><li>item1<ul><li>subitem11</li></ul></li><li>item2<ul><li>subitem21</li></ul></li></ul>

In other words: nothing is selected.