Почему в CLOS только standard-class является экземпляром самого себя?

(defclass abstract-class-mixin (standard-class)
  ((abstract :initform nil
             :initarg :abstract
             :reader abstract-p)))

(defmethod validate-superclass ((sc abstract-class-mixin) (c standard-class))
  t)

(defmethod validate-superclass ((c standard-class) (sc abstract-class-mixin))
  t)

(defclass std-class (abstract-class-mixin)
  ()
  (:metaclass abstract-class-mixin))

(defmethod allocate-instance ((class std-class) &rest args)
  (declare (ignore args))
  (if (abstract-p class)
    (error "Unable to allocate instance for abstract class ~s" (class-name class))
    (call-next-method)))

(defclass abstract-class (std-class)
  ()
  (:metaclass std-class)
  (:default-initargs :abstract t))

(defclass std-object ()
  ()
  (:metaclass std-class))

(defmacro define-class (name (&rest scs) (&rest slots) &rest opts)
  `(defclass ,name (,@scs std-object)
     ,slots
     ,@opts
     (:metaclass std-class)))

(defmacro define-abstract-class (name (&rest scs) (&rest slots) &rest opts)
  `(defclass ,name (,@scs std-object)
     ,slots
     ,@opts
     (:metaclass abstract-class)))

(define-abstract-class my-abstract-object ()
  ((slot :initform 'value)))

(define-class my-object (my-abstract-object)
  ())

;; CL-USER> (make-instance 'my-abstract-object)
;;   ==>
;; Unable to allocate instance for abstract class MY-ABSTRACT-OBJECT
;; [Condition of type SIMPLE-ERROR]

;; CL-USER> (describe (make-instance 'my-object))
;;   ==>
;; #<MY-OBJECT {258CF469}>
;;   [standard-object]
;; 
;; Slots with :INSTANCE allocation:
;;   SLOT  = VALUE

(defclass singleton-class () ;;метакласс, реализующий механизм синглтона
  ((instance :initform nil)))
 
(defmethod validate-superclass ((class singleton-class) (superclass standard-class))
  t) ;;Разрешаем наследование классов-синглтонов от обычных классов
 
(defmethod validate-superclass ((class singleton-class) (superclass singleton-class))
  t) ;;Разрешаем наследование классов-синглтонов от других классов-синглтонов
 
(defmethod validate-superclass ((class standard-class) (superclass singleton-class))
  nil) ;;Запрещаем наследование обычных классов от синглтонов
 
(defmethod make-instance ((class singleton-class) &key) 
  (with-slots (instance) class
    (or instance (setf instance (call-next-method)))))
 
(defclass my-singleton-class () 
  () 
  (:metaclass singleton-class))

class Singleton(type):
     def __init__(cls, name, bases, dict):
         super(Singleton, cls).__init__(name, bases, dict)
         cls.instance = None
     def __call__(cls,*args,**kw):
         if cls.instance is None:
             cls.instance = super(Singleton, cls).__call__(*args, **kw)
         return cls.instance
 
>>> class MyClass(object):
...     __metaclass__ = Singleton

(defclass c (standard-class) ())

(defclass c (standard-class) () (:metaclass c))

> (class-of (find-class 'c))
#<STANDARD-CLASS C>

Invalid initialization arguments:
  :DIRECT-DEFAULT-INITARGS, SB-PCL::SAFE-P, :DEFINITION-SOURCE,
  :DIRECT-SLOTS, :DIRECT-SUPERCLASSES, :NAME
in call for class #<STANDARD-CLASS SINGLETON-CLASS>.
   [Condition of type SB-PCL::INITARG-ERROR]

Restarts:
 0: [RETRY] Retry SLIME interactive evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [ABORT] Abort thread (#<THREAD "worker" RUNNING {100CF6E743}>)

Backtrace:
  0: (SB-PCL::CHECK-MI-INITARGS #<STANDARD-CLASS SINGLETON-CLASS> (:NAME MY-SINGLETON-CLASS :DIRECT-SUPERCLASSES NIL :DIRECT-SLOTS NIL :DEFINITION-SOURCE #S(SB-C:DEFINITION-SOURCE-LOCATION :NAMESTRING NIL ..
  1: ((:METHOD MAKE-INSTANCE (CLASS)) #<STANDARD-CLASS SINGLETON-CLASS> #<unavailable &REST argument>) [fast-method]
  2: ((FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK :IN SB-MOP:ENSURE-CLASS-USING-CLASS))
  3: ((FLET #:WITHOUT-INTERRUPTS-BODY-564 :IN SB-THREAD::CALL-WITH-RECURSIVE-LOCK))
  4: (SB-THREAD::CALL-WITH-RECURSIVE-LOCK #<CLOSURE (FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK :IN SB-MOP:ENSURE-CLASS-USING-CLASS) {82FFFEC9B}> #<SB-THREAD:MUTEX "World Lock" owner: #<SB-THREAD:THREAD "wo..
  5: ((:METHOD SB-MOP:ENSURE-CLASS-USING-CLASS (NULL T)) NIL MY-SINGLETON-CLASS :METACLASS SINGLETON-CLASS :DIRECT-SUPERCLASSES NIL :DIRECT-SLOTS NIL :DEFINITION-SOURCE #S(SB-C:DEFINITION-SOURCE-LOCATION :..
  6: ((FLET #:WITHOUT-INTERRUPTS-BODY-564 :IN SB-THREAD::CALL-WITH-RECURSIVE-LOCK))
  7: (SB-THREAD::CALL-WITH-RECURSIVE-LOCK #<CLOSURE (FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK :IN SB-MOP:ENSURE-CLASS) {82FFFEEFB}> #<SB-THREAD:MUTEX "World Lock" owner: #<SB-THREAD:THREAD "worker" RUNNIN..
  8: (SB-MOP:ENSURE-CLASS MY-SINGLETON-CLASS :METACLASS SINGLETON-CLASS :DIRECT-SUPERCLASSES NIL :DIRECT-SLOTS NIL :DEFINITION-SOURCE #S(SB-C:DEFINITION-SOURCE-LOCATION :NAMESTRING NIL :TOPLEVEL-FORM-NUMBE..
  9: (SB-PCL::REAL-LOAD-DEFCLASS MY-SINGLETON-CLASS SINGLETON-CLASS NIL NIL (:DIRECT-DEFAULT-INITARGS NIL) NIL NIL NIL #S(SB-C:DEFINITION-SOURCE-LOCATION :NAMESTRING NIL :TOPLEVEL-FORM-NUMBER 0 :PLIST NIL)..
 10: (SB-INT:SIMPLE-EVAL-IN-LEXENV (LET NIL (SB-PCL::LOAD-DEFCLASS (QUOTE MY-SINGLETON-CLASS) (QUOTE SINGLETON-CLASS) (QUOTE NIL) (LIST) (LIST :DIRECT-DEFAULT-INITARGS NIL) (QUOTE NIL) (QUOTE NIL) (QUOTE N..
 11: (SB-INT:SIMPLE-EVAL-IN-LEXENV (EVAL-WHEN (:LOAD-TOPLEVEL :EXECUTE) (LET NIL (SB-PCL::LOAD-DEFCLASS (QUOTE MY-SINGLETON-CLASS) (QUOTE SINGLETON-CLASS) (QUOTE NIL) (LIST) (LIST :DIRECT-DEFAULT-INITARGS ..
 12: (SB-INT:SIMPLE-EVAL-IN-LEXENV (DEFCLASS MY-SINGLETON-CLASS NIL NIL (:METACLASS SINGLETON-CLASS)) #<NULL-LEXENV>)
 13: (EVAL (DEFCLASS MY-SINGLETON-CLASS NIL NIL (:METACLASS SINGLETON-CLASS)))
 14: ((LAMBDA NIL :IN SWANK:INTERACTIVE-EVAL))
 --more--

(asdf:oos 'asdf:load-op :closer-mop)
(in-package :closer-mop)

(defclass singleton-class (standard-class) ;;метакласс, реализующий механизм синглтона
  ((instance :initform nil)))

(defmethod validate-superclass ((class singleton-class) (superclass standard-class))
  t) ;;Разрешаем наследование классов-синглтонов от обычных классов

(defmethod validate-superclass ((class singleton-class) (superclass singleton-class))
  t) ;;Разрешаем наследование классов-синглтонов от других классов-синглтонов

(defmethod validate-superclass ((class standard-class) (superclass singleton-class))
  nil) ;;Запрещаем наследование обычных классов от синглтонов

(defmethod make-instance ((class singleton-class) &key &allow-other-keys)
  (with-slots (instance) class
    (or instance (setf instance (call-next-method)))))

(defclass my-singleton-class ()
  ()
  (:metaclass singleton-class))