Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

Q: euclid's lemma, also called euclid's division lemma or euclid's first theorem, is an important lemma.

A: euclid's lemma
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Q: a video projector takes a video signal and projects the corresponding image on a screen using a lens system.

A: video projector
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Q: a sovereign state is a state with borders where people live, and where a government makes laws and talks to other sovereign states.

A:
sovereign state
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