Formal Notation
The disjunction introduction rule may be written in sequent notation:
where is a metalogical symbol meaning that is a syntactic consequence of in some logical system;
and expressed as a truth-functional tautology or theorem of propositional logic:
where and are propositions expressed in some formal system.
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“On every formal visit a child ought to be of the party, by way of provision for discourse.”
—Jane Austen (1775–1817)