🤡 Reductio ad absurdum §

= if A leads (=reductio) to a ❌ Contradiction (= absurdum), one can assume its negation (i.e. reversal of Negation (minimal logic))

Notation §

(KR)\frac{\begin{array} \\ [\neg A]_{i} \\ \vdots \\ \bot \end{array}}{A}i