diff --git a/checking.lisp b/checking.lisp
index f71ba2b..97c0aae 100644
--- a/checking.lisp
+++ b/checking.lisp
@@ -1,6 +1,6 @@
 (in-package #:definitions-systems)
 
-(defclass defsys:check-definition-mixin () ())
+(defclass defsys:check-definition-mixin (defsys:system) ())
 
 (defgeneric defsys:base-definition-class (system))
 
diff --git a/definitions.lisp b/definitions.lisp
index 32a9660..8df4f7d 100644
--- a/definitions.lisp
+++ b/definitions.lisp
@@ -31,7 +31,7 @@
         (error "Don't know how to ~S for definition ~S because it has no owner."
                'make-load-form definition))))
 
-(defclass defsys:primary-binding-mixin (defsys:owner-mixin defsys:name-mixin)
+(defclass defsys:primary-binding-mixin (defsys:owner-mixin defsys:name-mixin defsys:definition)
   ())
 
 (defmethod shared-initialize :after ((definition defsys:primary-binding-mixin) slot-names
@@ -52,7 +52,7 @@
 (defmethod (setf defsys:name) (new-name (definition defsys:primary-binding-mixin))
   (reinitialize-instance definition :name new-name))
 
-(defclass defsys:alias-bindings-mixin ()
+(defclass defsys:alias-bindings-mixin (defsys:definition)
   ((%aliasing-systems :reader %aliasing-systems
                       :type hash-table
                       :initform (make-hash-table :test 'eq))))
diff --git a/expansion.lisp b/expansion.lisp
index 77d0032..4169df1 100644
--- a/expansion.lisp
+++ b/expansion.lisp
@@ -1,6 +1,6 @@
 (in-package #:definitions-systems)
 
-(defclass defsys:default-definition-class-mixin ()
+(defclass defsys:default-definition-class-mixin (defsys:system)
   ((%default-definition-class :initarg :default-definition-class
                               :type (or null class)
                               :initform nil)))
diff --git a/systems.lisp b/systems.lisp
index cd1527a..de2c31e 100644
--- a/systems.lisp
+++ b/systems.lisp
@@ -2,13 +2,13 @@
 
 (defclass defsys:system () ())
 
-(defclass defsys:hash-table-mixin ()
+(defclass defsys:hash-table-mixin (defsys:system)
   ((%hash :reader %hash
           :type hash-table
           :initform (make-hash-table :test 'eq))))
 
-(defclass defsys:standard-system (defsys:system
-                                  defsys:base-definition-class-mixin
+(defclass defsys:standard-system (defsys:base-definition-class-mixin
                                   defsys:standard-definition
-                                  defsys:hash-table-mixin)
+                                  defsys:hash-table-mixin
+                                  defsys:system)
   ())