Proof by Cases/Proof Rule/Tableau Form

Proof Rule
Let $\phi \lor \psi$ be a compound statement form in a tableau proof whose main connective is the disjunction operator.

Let $\chi$ be a statement form such that $\left({\phi \vdash \chi}\right)$, $\left({\psi \vdash \chi}\right)$.

The Rule of Or-Elimination is invoked for $\phi \lor \psi$ as follows: