Tableau Extension Lemma/General Statement/Proof 1
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Theorem
Let $T$ be a finite propositional tableau.
Let its hypothesis set $\mathbf H$ be finite.
Let $\mathbf H'$ be another finite set of WFFs.
Then there exists a finished finite propositional tableau $T'$ such that:
$(1):\quad$ the root of $T'$ is $\mathbf H \cup \mathbf H'$;
$(2):\quad$ $T$ is a rooted subtree of $T'$.
Proof
Let $T_{\mathbf H'}$ be the finite propositional tableau obtained by replacing the hypothesis set $\mathbf H$ of $T$ with $\mathbf H \cup \mathbf H'$.
By the Tableau Extension Lemma, $T_{\mathbf H'}$ has a finished extension $T'$.
By definition of extension, $T_{\mathbf H'}$ is a rooted subtree of $T'$.
But $T_{\mathbf H'}$ and $T$ are equal when considered as rooted trees.
The result follows.
$\blacksquare$