CL-DIFFLIB
CL-DIFFLIB is a Lisp library for computing differences between sequences. It is based on the Python difflib module.

CL-DIFFLIB is copyright John Wiseman and is covered by the MIT license.

See this lemonodor post for more information.

You may also be interested in Nathan Froyd's diff package (though it seems to support only SBCL).

Download: http://lemonodor.com/code/cl-difflib_0.1.tar.gz