fall in

1. break down, literally or metaphorically; "The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice"

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2. to take one's place in a military formation or line; "Troops fall in!"

3. become part of; become a member of a group or organization; "He joined the Communist Party as a young man"

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