  Many properties of communication protocols combine safety and liveness
aspects. Characterizing such combined properties by means of a single inference
system is difficult because of the fundamentally different techniques
(coinduction and induction, respectively) usually involved in defining and
proving them. In this paper we show that Generalized Inference Systems allow us
to obtain sound and complete characterizations of (at least some of) these
combined inductive/coinductive properties of binary session types. In
particular, we illustrate the role of corules in characterizing fair
termination (the property of protocols that can always eventually terminate),
fair compliance (the property of interactions that can always be extended to
reach client satisfaction) and fair subtyping, a liveness-preserving refinement
relation for session types. The characterizations we obtain are simpler
compared to the previously available ones and corules provide insight on the
liveness properties being ensured or preserved. Moreover, we can conveniently
appeal to the bounded coinduction principle to prove the completeness of the
provided characterizations.
