But one day Banzo crept up behind him and gave him a terrific blow with a wooden sword.

但有一天，Banzo悄悄走到他身后，用木剑重重击了他一下。

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Kojiro was enraged that Musashi would be late, and that he would challenge him with only a wooden sword.

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They have seen their "oil weapon" exposed as a wooden sword.

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Another possibility is that Musashi deliberately arrived late to disconcert his opponent, used only a wooden sword to further unbalance him, and used the psychological advantage he created, along with his formidable swordsmanship, to win the duel.

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