CommonLispObjectSystem (aka. CLOS)

The standardized object-oriented extension of CommonLisp.

See e.g.:

• ​http://en.wikipedia.org/wiki/Common_Lisp_Object_System
• ​http://www.aiai.ed.ac.uk/~jeff/clos-guide.html
• ​http://www.lisp.org/table/objects.htm