with respect to namespaces:
>>> from lxml import etree
>>> doc = etree.parse('in.html')
>>> names = {'cas':'http:///text/cas.ecore', 'xmi': 'http://blahblah'}
>>> doc.xpath('//cas:Sofa[@xmi:id="63"]', namespaces=names)
[<Element {http:///text/cas.ecore}Sofa at 0x10550a5f0>]
>>> doc.xpath('//cas:Sofa[@xmi:id="63"]/@String', namespaces=names)
['Find a contact']
>>> doc.xpath('//cas:Sofa[@xmi:id="72" and @ID="Third"]/@String', namespaces=names)
['Send a message']