Interface:Law of the excluded middle

To state the axiom, we need formulas including disjunction and negation. The parameter here can be any flavor of propositional logic which defines them, including intuitionistic or classical logic.  param (PROPOSITIONAL Interface:Classical_propositional_calculus )

var (formula p) 

Here is the axiom:  stmt (TertiumNonDatur  (p ∨ (¬ p))) 

Equivalent axioms
Assuming Interface:Intuitionistic propositional logic, a variety of axioms all have the effect of producing classical propositional logic. The law of the excluded middle is one of them. A few others are:  double negation elimination ¬ ¬ p → p transposition elimination (¬ q → ¬ p) → (p → q) case elimination (p → q) ∧ (¬ p → q) → q</dd> Peirce's law</dt> ((p → q) → p) → p</dd> </dl>