  Inspired by Kobayashi's type system for lock freedom, we define a behavioral
type system for ensuring progress in a language of binary sessions. The key
idea is to annotate actions in session types with priorities representing the
urgency with which such actions must be performed and to verify that processes
perform such actions with the required priority. Compared to related systems
for session-based languages, the presented type system is relatively simpler
and establishes progress for a wider range of processes.
