 Probabilities belonging to a given subset: Ognjanović and Rašković (1999) extend the language of probability logic by means of a new type of operator: \(Q_F\). Intuitively, the formula \(Q_F\phi\) means that the probability of \(\phi\) belongs to \(F\), for some given set \(F \subseteq [0,1]\). This \(Q_F\)-operator cannot be defined in terms of formulas of the form \(P(\phi) \ge a\). Ognjanović and Rašković (1999) provide a sound and complete axiomatization of this type of logical system. The key bridge principles, which connect the \(Q_F\)-operator to the more standard \(P\)-operator, are the axioms \(P(\phi) = a \to Q_F\phi\) for all \(a \in F\), as well as the infinitary rule that specifies that from \(P(\phi) = a \to \psi\) for all \(a \in F\), one can infer \(Q_F\phi\to\psi\).