  Extending and generalizing the approach of 2-sequents (Masini, 1992), we
present sequent calculi for the classical modal logics in the K, D, T, S4
spectrum. The systems are presented in a uniform way-different logics are
obtained by tuning a single parameter, namely a constraint on the applicability
of a rule. Cut-elimination is proved only once, since the proof goes through
independently from the constraints giving rise to the different systems. A
sequent calculus for the discrete linear temporal logic ltl is also given and
proved complete. Leitmotiv of the paper is the formal analogy between modality
and first-order quantification.
