Back and forth method is a new tool in study of model theory and its application. This paper construct short proofs for two theorems in modal logic by back and forth method.

向前向后方法是模型论及其应用研究中的一个新的工具。本文使用向前向后方法，对模态逻辑的两个定理构造了一种简单的证明。

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Back and forth method is a new tool in study of model theory and its application.

向前向后方法是模型论及其应用研究中的一个新的工具。

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Back-and-forth method is an important tool for studying model theory and its application.

向前向后方法是模型论及其应用研究中的一个很重要的工具。

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Hence the 3 email rule which states if the email back and forth goes to 3 emails break the chain and try another method such as a phone call or a face-to-face meeting.

FORBES: Better Communication: Technology Isn't Always The Best Solution