In modern systems of law, formal requirements invariably call for some kind of written instrument.

在现代法律体系中, 形式要件要求有某种书面文件上的规定总是需要一些书面文书.

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This Contract shall not be changed verbally, but a written instrument signed by the Parties.

合同不得以口头方式修改, 而须经双方签署书面文件后方可修改.

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