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to happen to be present
事故現場に居合わせた目撃者。
A witness who happened to be at the accident scene
たまたま居合わせて、事故の一部始終を目撃した。
I happened to be there and witnessed the entire accident.
現場に居合わせている人に話を聞いた。
I spoke with people who were present at the scene.