Comment 4 for bug 1324972

This all looks acceptable.

I'm not fussed if the new API is only available with modern python, so we can stick with namedtuple and collections, but might need to drop the 'with' syntax to keep the file parseable.

I'm not thrilled about the iterator passed to LazyList keeeping an open file handle, even if never used. Defining the iterator as a class with the magic iterator methods should make it lazier, not attempting to open the file until it is needed. I certainly don't want the new code to fail on import if zone.tab is missing.