One example I can think of is how Ring middleware works. The request map is passed through a series of functions, and each one can modify it in its own way. For

One example I can think of is how Ring middleware works. The request map is passed through a series of functions, and each one can modify it in its own way. For example, you could have an authentication library that adds an :identity key, and another that adds a :session key. These libraries can work together without having to know anything about each other.