ChangeSafe is a complex piece of software. Revision control and configuration management is tricky enough, but we wanted to solve an even more difficult problem, that of making revision control and configuration management look easy. To do this, we needed a new computer language: one in which versioned data structures are built in, one that allows us to switch our viewpoint from a single instant of time to the entire history of the stored data, one in which the primitive operations of assignment and object creation can themselves be reflected back into the language as objects to be manipulated. Of course this new language needs all the usual features --- numbers, strings, arithmetic, procedures etc. --- and we'll make heavy use of some advanced features --- multi-threading, structured error handling, reflection --- and we want a rich set of auxiliary libraries --- collections, web stuff like sockets, http, and html generation, database connectivity. We can't forget the infrastructure tools like debuggers, profilers, and a reasonable IDE. But metalinguistic abstraction --- inventing custom computer languages --- doesn't mean that you have to start from scratch. Some computer languages make it easy and natural to extend the core language with new language constructs. That is why, with the exception of the little Java applet that runs on the client machine, all of ChangeSafe is written in Common Lisp.