+@item
+@findex @cl{defclass}
+@findex @sbmop{ensure-class}
+@findex @sbmop{ensure-class-using-class}
+@findex @cl{find-class}
+@findex @cl{class-name}
+the class named by the @code{name} argument to @code{ensure-class}, if
+any, is only redefined if it is the proper name of that class;
+otherwise, a new class is created.
+
+This is consistent with the description of @code{ensure-class} in AMOP
+as the functional version of @code{defclass}, which has this behaviour;
+however, it is not consistent with the weaker requirement in AMOP, which
+states that any class found by @code{find-class}, no matter what its
+@code{class-name}, is redefined.
+
+@end itemize
+
+@subsection Metaobject Protocol Extensions
+
+In addition, SBCL supports extensions to the Metaobject protocol from
+AMOP; at present, they are:
+
+@itemize
+
+@item
+@findex @cl{defmethod}
+@findex @cl{find-class}
+@findex @sbmop{intern-eql-specializer}
+@findex @sbpcl{make-method-specializers-form}
+@findex @sbmop{make-method-lambda}
+compile-time support for generating specializer metaobjects from
+specializer names in @code{defmethod} forms is provided by the
+@code{make-method-specializers-form} function, which returns a form
+which, when evaluated in the lexical environment of the
+@code{defmethod}, returns a list of specializer metaobjects. This
+operator suffers from similar restrictions to those affecting
+@code{make-method-lambda}, namely that the generic function must be
+defined when the @code{defmethod} form is expanded, so that the
+correct method of @code{make-method-specializers-form} is invoked.
+The system-provided method on @code{make-method-specializers-form}
+generates a call to @code{find-class} for each symbol specializer
+name, and a call to @code{intern-eql-specializer} for each @code{(eql
+@var{x})} specializer name.
+
+@item
+@findex @cl{find-method}
+@findex @sbpcl{parse-specializer-using-class}
+@findex @sbpcl{unparse-specializer-using-class}
+run-time support for converting between specializer names and
+specializer metaobjects, mostly for the purposes of
+@code{find-method}, is provided by
+@code{parse-specializer-using-class} and
+@code{unparse-specializer-using-class}, which dispatch on their first
+argument, the generic function associated with a method with the given
+specializer. The system-provided methods on those methods convert
+between classes and proper names and between lists of the form
+@code{(eql @var{x})} and interned eql specializer objects.
+
+@item
+@vindex @sbpcl{+slot-unbound+}
+@findex @sbmop{standard-instance-access}
+@findex @sbmop{funcallable-standard-instance-access}
+distinguising unbound instance allocated slots from bound ones when
+using @code{standard-instance-access} and
+@code{funcallable-standard-instance-access} is possible by comparison
+to the constant @code{+slot-unbound+}.
+