Replace the word "theory" with "abstraction" or "interface" then. All the article is saying is that there's a bunch of common things (Unix pipes, function compo

Replace the word "theory" with "abstraction" or "interface" then. All the article is saying is that there's a bunch of common things (Unix pipes, function composition, procedure sequencing) that all follow a few simple rules (a "contract" if you like). This means you can reason about them together without having to think about the individual "implementation details".