Hybrid logic refers to a number of extensions to propositional modal logic with more expressive power, though still less than first-order logic. In formal logic, there is a trade-off between expressiveness and computational tractability (how easy it is to compute/reason with logical languages). The history of hybrid logic began with Arthur Prior's work in tense logic. @i p is true if and only if p is true in the unique state named by the nominal i (i.e., the state where i is true). Hybrid logics with extra or other operators exist, but @ is more-or-less "standard."