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.

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

nelsan ellis (november 30, 1977 – july 8, 2017) was an american actor and playwright.
nelsan ellis

tuff (from italian "tufo") is a type of rock produced by the eruption of volcanoes.

tuff