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.

Example Input: a sandwich panel is a kind of composite material, which is usually made of two components: a small outside layer (which is the same on both sides), and an inside layer, which is used to get certain properties, for example insulation or fire resistance.
Example Output: sandwich panel

Example Input: the iraq national under-23 football team are a team who play association football for iraq at the under-23 level.
Example Output: iraq national under-23 football team

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