 A propositional i-form can therefore be defined as the \(i\)-form of at least one proposition \(s\); and a propositional form is a propositional \(i\)-form with respect to at least one sequence \(i\). Due to the proviso mentioned, a propositional form as defined before can never be empty or a singleton. We can now define universal validity and universal contravalidity first for a propositional form and subsequently for a proposition in the following way: