In this paper, we present Bast, an extensible library of protocol classes. The latter is aimed at helping system programmers to build distributed programming environments. Protocol classes constitute the basic structuring components for higher-level programming models, such as the transactional model, and add flexibility to distributed environments. We focus on classes that implement a generic agreement protocol named DTM (Dynamic-Terminating-Multicast). To the programmer, the DTM generic protocol appears as a set of classes that can be specialized to solve agreement problems in distributed systems. In particular, we show how those classes can be derived to build atomic commitment and reliable total order protocols. An overview of the Smalltalk design and implementation of the Bast library is also presented.