A heading in a book is just as much a contract about the content as a method signature is about the content of a method. It takes effort and creativity to produ

A heading in a book is just as much a contract about the content as a method signature is about the content of a method. It takes effort and creativity to produce a coherent outline for a book. And as any (competent) programmer knows, the "interface" in "code against interfaces not implementations" is merely a homonym of the "interface" in "application programming interface".