_(CLOS) is the Common Lisp Object System, an object-oriented *(programming language) embedded in and somewhat integrated into _(Common Lisp).

The <a href="http://www-2.cs.cmu.edu/Groups/AI/html/cltl/clm/node260.html#SECTION003200000000000000000">specification</a>.

<a href="http://www.cetus-links.org/oo_clos.html">Some links</a>.
