This video is demonstrating a design pattern that could be used to solve a variety of problems in Common Lisp. By utilizing macros we create a DSL that can be used to define commands and their argument validators that a user would be able to interact with, we also demonstrate a way you can use conditions to provide useful information to the user.
It should be noted that I am using (print-object ..) to pretty print conditions, but really the :report keyword should be used see http://www.lispworks.com/documentatio... for more information.