Comment 2 for bug 401618

The hardest case I ran into when I looked into this was in lib/lp/translations/browser/configure.zcml, which uses path_expression="string:${sequence}" to navigate from a POFile to a TranslationMessage.

I tried wrapping the TranslationMessage as used in browser code in a little "TM-in-POFile" class that delegates ITranslationMessage to the TranslationMessage but also remembers the message's sequence number in the given POFile. That patch grew very quickly though, and also replaced TranslationMessage.pofile, setPOFile, and browser_pofile.