Rule of Idempotence/Disjunction
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Theorem
The disjunction operation is idempotent:
Formulation 1
- $p \dashv \vdash p \lor p$
Formulation 2
- $\vdash p \iff \paren {p \lor p}$
Its abbreviation in a tableau proof is $\textrm{Idemp}$.
Also known as
This rule is also known as the law of tautology for logical addition (or disjunction).