Rules of inference for exclusive disjunction and logical biconditional

logicnatural-deductionpropositional-calculussequent-calculus

Here are the rules of inference in natural calculus of propositions. I'd like to extend this calculus (conservatively) adding exclusive disjunction $\oplus$ and logical biconditional $\sim$ and obtain a complete system. There are many ways to describe their 'behavior', for instance, 2 rules:

$$\cfrac{\Gamma,\neg A\vdash B;\quad \Gamma,\neg B\vdash A}{\Gamma\vdash A\oplus B};\qquad\cfrac{\Gamma\vdash A\oplus B}{\Gamma,A,B\vdash}$$

would be enough for strict disjunction.

My question is: which way is traditional (if there is such a tradition) and most convinient?

Best Answer

For bi-conditional, I would suggest:

$\dfrac {\Gamma, A \vdash B \ \ \ \ \Gamma, B \vdash A}{\Gamma \vdash A \leftrightarrow B} \text { for }(\leftrightarrow \text I)$

and the pair:

$\dfrac {\Gamma \vdash A \leftrightarrow B}{\Gamma, A \vdash B} \ \ \ \dfrac {\Gamma \vdash A \leftrightarrow B}{\Gamma, B \vdash A} \text { for }(\leftrightarrow \text E)$


For exclusive or, the pair:

$\dfrac {\Gamma, A \vdash \ \ \ \ \Gamma \vdash B}{\Gamma \vdash A \oplus B} \ \ \ \dfrac {\Gamma, \vdash A \ \ \ \ \Gamma, B \vdash}{\Gamma \vdash A \oplus B} \text { for }(\oplus \text I)$

and the corresponding for $(\oplus \text E)$, based on the fact that an $\oplus$ True formula can produce cases: either $A$ False and $B$ True or the symmetric one:

$\dfrac {\Gamma, A \vdash \ \ \ \ \Gamma \vdash A \oplus B}{ \Gamma \vdash B} \ \ \ \dfrac {\Gamma, \vdash A \ \ \ \ \Gamma \vdash A \oplus B}{\Gamma, B \vdash}$

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