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https://books.google.co.kr/books?id=3_ILCwAAQBAJ&pg=PP4&lpg=PP4&dq=tridoku&source=bl&ots=YD1LRHSvHP&sig=ACfU3U0wBJumNSSGwkJ6gtwoMkngfWQeiQ&hl=ko&sa=X&ved=2ahUKEwiBtPiTgbTnAhVCfd4KHVAXAq4Q6AEwD3oECAcQAQ#v=onepage&q=tridoku&f=false
