  We provide here a simple, yet very general framework that allows us to
explain several constraint propagation algorithms in a systematic way. In
particular, using the notions commutativity and semi-commutativity, we show how
the well-known AC-3, PC-2, DAC and DPC algorithms are instances of a single
generic algorithm. The work reported here extends and simplifies that of Apt,
cs.AI/9811024.
