Source code from the April 2007 meeting of LispSea: Lisp manipulation of C structures
This includes full source code beyond what has been contributed to WikiBooks.
Much thanks to the person with deep knowledge of SBCL internals who elaborated on the point that characters are not cheap in ANSI CL as someone with a C/C++ or Perl/Python/Ruby background might assume. There are font and other metadata associated with them. Also, your Lisp system of choice may not necessarily use Unicode, iso-8859-1 or even ASCII encoding. Additionally, some Lisp systems may choke on non-ASCII values, specifically anything above 127.
See also: Trivial UTF-8.