Yes, my notice is half valid. The is necessary for the actions, not for
storing. Anyway, not a big difference in efficiency. :-) I mean, adding to
set works with a builtin "if" that is slightly faster than an interpreted if
line, and anyway, it will be executed.
seenCategories.add(cat.title());
if not cat.title() in seenCategories:
# do something with this category