After reading your latest comment, it finally clicked for me that `webserver-service` is a *service* (duh), not a service function.  I don't know how I got confused about that.  Let's blame the vicodin.  So, yeah, it is more pluggable than I previously thought.

Perhaps this section would benefit from some example code?  It seems like a lot of text about concepts and such ... some example code might be in order for it to grab the reader. (like an example of defining multiple "implementations of a contract")