Finished Branch Lemma

Theorem
Let $\Gamma$ be a finished branch of a propositional tableau $\left({T, \mathbf H, \Phi}\right)$.

Let $v$ be a boolean interpretation such that:


 * $v \models_{\mathrm{BI}} \mathbf A$ for every basic WFF $\mathbf A$ that occurs along $\Gamma$.

Then:


 * $v \models_{\mathrm{BI}} \Phi \left[{\Gamma}\right]$

where $\Phi \left[{\Gamma}\right]$ is the image of $\Gamma$ under $\Phi$.

Proof
The proof appeals to the Principle of Structural Induction, applied to the statement:


 * If $\mathbf C$ occurs along $\Gamma$, then $v \models_{\mathrm{BI}} \mathbf C$.

When $\mathbf C$ is basic, the result holds per assumption.

Suppose $\mathbf C$ is not basic.

It is seen that one of the propositional tableau construction rules applies to $\mathbf C$.

If $\mathbf C = \neg\neg\mathbf A$, then it can only be used by the $\boxed{\neg\neg}$ rule.

Hence, $\mathbf A \in \Phi \left[{\Gamma}\right]$, and by induction hypothesis, $v \models_{\mathrm{BI}} \mathbf A$.

By Double Negation, $v \models_{\mathrm{BI}} \neg\neg\mathbf A$.

If $\mathbf C$ is $\mathbf A \land \mathbf B$, then it can only be used by the $\boxed\land$ rule.

Therefore, $\mathbf A, \mathbf B \in \Phi \left[{\Gamma}\right]$, and by induction hypothesis:


 * $v \models_{\mathrm{BI}} \mathbf A,\mathbf B$

By the truth function for $\land$, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A \land \mathbf B$

If $\mathbf C$ is $\neg \left({\mathbf A \land \mathbf B}\right)$, then it can only be used by the $\boxed{\neg\land}$ rule.

Therefore, $\neg\mathbf A \in \Phi \left[{\Gamma}\right]$ or $\neg\mathbf B \in \Phi \left[{\Gamma}\right]$.

By induction hypothesis, it follows that:


 * $v \models_{\mathrm{BI}} \neg\mathbf A$ or $v \models_{\mathrm{BI}} \neg\mathbf B$

By the truth function for $\land$, it follows that:


 * $v \models_{\mathrm{BI}} \neg \left({\mathbf A \land \mathbf B}\right)$

If $\mathbf C$ is $\mathbf A \lor \mathbf B$, then it can only be used by the $\boxed\lor$ rule.

Therefore, $\mathbf A \in \Phi \left[{\Gamma}\right]$ or $\mathbf B \in \Phi \left[{\Gamma}\right]$.

By induction hypothesis, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A$ or $v \models_{\mathrm{BI}} \mathbf B$

By the truth function for $\lor$, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A \lor \mathbf B$

If $\mathbf C$ is $\neg \left({\mathbf A \lor \mathbf B}\right)$, then it can only be used by the $\boxed{\neg\lor}$ rule.

Therefore, $\neg\mathbf A, \neg\mathbf B \in \Phi \left[{\Gamma}\right]$, and by induction hypothesis:


 * $v \models_{\mathrm{BI}} \neg\mathbf A,\neg\mathbf B$

By the truth function for $\lor$, it follows that:


 * $v \models_{\mathrm{BI}} \neg \left({\mathbf A \lor \mathbf B}\right)$

If $\mathbf C$ is $\mathbf A \implies \mathbf B$, then it can only be used by the $\boxed\implies$ rule.

Therefore, $\neg\mathbf A \in \Phi \left[{\Gamma}\right]$ or $\mathbf B \in \Phi \left[{\Gamma}\right]$.

By induction hypothesis, it follows that:


 * $v \models_{\mathrm{BI}} \neg\mathbf A$ or $v \models_{\mathrm{BI}} \mathbf B$

By the truth function for $\implies$, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A \implies \mathbf B$

If $\mathbf C$ is $\neg \left({\mathbf A \implies \mathbf B}\right)$, then it can only be used by the $\boxed{\neg\implies}$ rule.

Therefore, $\mathbf A, \neg\mathbf B \in \Phi \left[{\Gamma}\right]$, and by induction hypothesis:


 * $v \models_{\mathrm{BI}} \mathbf A,\neg\mathbf B$

By the truth function for $\implies$, it follows that:


 * $v \models_{\mathrm{BI}} \neg \left({\mathbf A \implies \mathbf B}\right)$

If $\mathbf C$ is $\mathbf A \iff \mathbf B$, then it can only be used by the $\boxed\iff$ rule.

Therefore, $\mathbf A\land \mathbf B \in \Phi \left[{\Gamma}\right]$ or $\neg\mathbf A \land \neg\mathbf B \in \Phi \left[{\Gamma}\right]$.

By induction hypothesis, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A,\mathbf B$ or $v \models_{\mathrm{BI}} \neg\mathbf A,\neg\mathbf B$

By the truth function for $\iff$, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A \iff \mathbf B$

If $\mathbf C$ is $\neg \left({\mathbf A \iff \mathbf B}\right)$, then it can only be used by the $\boxed{\neg\iff}$ rule.

Therefore, $\mathbf A\land \neg\mathbf B \in \Phi \left[{\Gamma}\right]$ or $\neg\mathbf A \land \mathbf B \in \Phi \left[{\Gamma}\right]$.

By induction hypothesis, it follows that:


 * $v \models_{\mathrm{BI}} \mathbf A,\neg\mathbf B$ or $v \models_{\mathrm{BI}} \neg\mathbf A,\mathbf B$

By the truth function for $\iff$, it follows that:


 * $v \models_{\mathrm{BI}} \neg \left({\mathbf A \iff \mathbf B}\right)$

Having dealt with all cases, it follows from the Principle of Structural Induction that:


 * $v \models_{\mathrm{BI}} \mathbf C$ for all WFFs $\mathbf C$ that occur along $\Gamma$

That is to say:


 * $v \models_{\mathrm{BI}} \Phi \left[{\Gamma}\right]$