In my current project, I'm working with Allegro CL in modern mode. Modern mode is a non-standard mode supporting mixed-case symbols in the reader. This means that in modern mode, the two literal symbols :foo and :FOO are different things. This is different from other lisps, that convert names of literal symbols read to uppercase before interning them. This is why :foo, :Foo and :FOO all mean the same :FOO, by default.
I am using the excellent Quicklisp package management utility to pull all dependencies that a certain package needs. With ACL in modern mode, though, I had to fix several libraries that used all-caps symbol literals (like #:FOO, NIL etc.). The fixes were mostly trivial, but when I had all the stuff running that I wanted, I had modified several of the libraries that I had installed with Quicklisp.
I am planning to import all the external libraries into git repositories and not use Quicklisp in production. I still want to be able to easily update to new upstream versions and also send the changes that I made myself upstream. To make the process of finding what I've changed easier, I envisioned a tool that tells me which of the libraries that I had installed with Quicklisp have local changes, and what these changes were. I asked Zach Beane for help, and he promptly came up with a small function called QDIFF that performs a diff between Quicklisp's base version and the locally extracted version of a library. I hacked the tool slightly to make it work with Allegro CL and iterate over all locally installed systems. It is available as a gist.
Zach supplied a SBCL specific way to run the diff command. I added one based on ASDF:RUN-SHELL-COMMAND, which apparently prefixes all output lines with "; ". I don't like that, but I also did not want to pull in a shell compatibility layer into this self-contained thing.
I praised Quicklisp in the past, but there is nothing wrong with doing it again. Open Source Common Lisp has become so much better with it, and I need to thank Zach again for being so passionate about making the Common Lisp world better! Thanks, Zach!