The most interesting part is the second page where he talks about contracts. One of the things that I observed (along with others) at Taligent was that there weren't enough ways to make contracts explicit because there were lots of different kinds of contracts. So here he's talking about incoming and outgoing contracts, which you could call client, and overrider contracts. But when you are building frameworks, the number of contracts doubles for some classes, because you have client and overrider contracts for "users" of the framework, and client and overrider contracts for the internals of the framework itself (given sufficiently complicated frameworks, which we had).