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by chance; by accident; accidentally; happen to (be, do); unexpectedly; casually
occasionally; once in a while; now and then
適同じ電車に乗り合わせて驚いた。
I was surprised to happen to be on the same train.
適通りかかった人に道を教えてもらった。
I happened to get directions from someone passing by.
適手元にあった本が役に立ったのだ。
A book that happened to be at hand turned out to be useful.