Clozure CL supports a fairly large subset of the semi-standard MetaObject Protocol (MOP) for CLOS, as defined in chapters 5 and 6 of "The Art Of The Metaobject Protocol", (Kiczales et al, MIT Press 1991, ISBN 0-262-61074-4); this specification is also available online at http://www.alu.org/mop/index.html.