unique elements from list of lists

[Tekkaman]

Thanks everybody!
Azrael: your suggestions involve python-level membership testing and
dummy list construction just like my uniter3 example, so I'm afraid
they would not go any faster. There's no built-in sequence flattening
function that I know of btw.
bearophile: your implementation is very similar to my uniter2
(subsequent set unions) but without the additional lambda def
overhead, and in fact it goes faster. Not faster than uniter though.
Peter: your solution is the fastest, removing the explicit for loop
resulted in a 30% speed gain. Looks like using set() is a must.

-- Simone