how to add an xml node based on the value of a text node

I am playing around with an xml file found @ http://www.jsphylosvg.com/examples/source.php?example=2&t=xml

I would like to insert a node if the value of the node name="Espresso".

E.g. I would like to change from:

<clade>
<name>Espresso</name>
<branch_length>2.0</branch_length>
</clade>

to:

<clade>
<name>Espresso</name>
<url>www.espresso.com</url>
<branch_length>2.0</branch_length>
</clade>

Based on the research I have done thus far I can use xpath to find the node that contains espresso (this should work, but it does not?)

import re, sys
import lxml.etree
f = open("test.xml", "r")
data = f.read()
tree = lxml.etree.XML(data)
if tree.xpath('//name/text()="Espresso"'):
    insert new child here

At this point, it should be possible to use use lxml.etree.Element to make the xml node, and use insert to attach them into xml document

However, while this sounds great in theory, I am unable to get it to work.
I would really appreciate any help/suggestions