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+Given a real Python class 'A', a wrapped C++ class 'B', and this definition:
+
+    class C(A, B):
+        def __init__(self):
+            B.__init__(self)
+            self.x = 1
+        ...
+
+    c = C()
+
+this diagram describes the internal structure of an instance of 'C', including
+its inheritance relationships. Note that ExtensionClass<B> is derived from
+Class<ExtensionInstance>, and is in fact identical for all intents and purposes.
+
+                                           MetaClass<ExtensionInstance>
+                 +---------+              +---------+                  
+types.ClassType: |         |              |         |                  
+                 |         |              |         |                  
+                 |         |              |         |                  
+                 +---------+              +---------+                    
+                   ^                         ^   ^
+     PyClassObject |       ExtensionClass<B> |   |
+ A: +------------+ |   B: +------------+     |   |
+    |  ob_type  -+-+      |   ob_type -+-----+   |
+    |            |   ()<--+- __bases__ |         |
+    |            |        |  __dict__ -+->{...}  |
+    |            |   'B'<-+- __name__  |         |
+    +------------+        +------------+         |
+         ^                      ^                |
+         |                      |                |
+         +-----+  +-------------+                |
+               |  |                              |
+               |  |     Class<ExtensionInstance> |
+               |  | C: +------------+            |
+               |  |    |   ob_type -+------------+
+        tuple:(*, *)<--+- __bases__ |           
+                       |  __dict__ -+->{__module__, <methods, etc.>}
+                 'C' <-+- __name__  |
+                       +------------+        
+                             ^           (in case of inheritance from more than one   
+                             |            extension class, this vector would contain  
+             +---------------+            a pointer to an instance holder for the data
+             |                            of each corresponding C++ class)            
+             |     ExtensionInstance                
+             | c: +---------------------+   std::vector<InstanceHolderBase> 
+             +----+- __class__          |  +---+--  
+                  |  m_wrapped_objects -+->| * | ...
+        {'x': 1}<-+- __dict__           |  +-|-+--   
+                  +---------------------+    |    InstanceValueHolder<B>                  
+                                             |   +--------------------------------+
+                                             +-->| (contains a C++ instance of B) |
+                                                 +--------------------------------+
+
+
+
+
+
+
+In our inheritance test cases in extclass_demo.cpp/test_extclass.py, we have the
+following C++ inheritance hierarchy:
+
+               +-----+          +----+
+               |  A1 |          | A2 |
+               +-----+          +----+
+                ^ ^ ^            ^  ^  
+                | | |            |  |
+          +-----+ | +---------+-----+
+          |       |           |  |
+          |       +---+----------+
+   .......!......     |       |
+   : A_callback :   +-+--+  +-+--+
+   :............:   | B1 |  | B2 |
+                    +----+  +----+
+                      ^
+                      |
+              +-------+---------+
+              |                 |
+            +-+-+         ......!.......
+            | C |         : B_callback :
+            +---+         :............:
+
+
+A_callback and B_callback are used as part of the wrapping mechanism but not
+represented in Python. C is also not represented in Python but is delivered
+there polymorphically through a smart pointer.
+
+This is the data structure in Python.
+
+       ExtensionClass<A1>
+  A1: +------------+                
+ ()<--+- __bases__ |                                                
+      |  __dict__ -+->{...}                                         
+      +------------+                                                
+        ^                 
+        |          ExtensionInstance                                              
+        |     a1: +---------------------+   vec     InstanceValueHolder<A1,A_callback> 
+        +---------+- __class__          |  +---+   +---------------------+ 
+        |         |  m_wrapped_objects -+->| *-+-->| contains A_callback | 
+        |         +---------------------+  +---+   +---------------------+              
+        |   
+        |          ExtensionInstance                                              
+        | pa1_a1: +---------------------+   vec     InstancePtrHolder<auto_ptr<A1>,A1>
+        +---------+- __class__          |  +---+   +---+ 
+        |         |  m_wrapped_objects -+->| *-+-->| *-+-+   A1    
+        |         +---------------------+  +---+   +---+ |  +---+
+        |                                                +->|   |
+        |          ExtensionInstance                        +---+ 
+        | pb1_a1: +---------------------+   vec     InstancePtrHolder<auto_ptr<A1>,A1>
+        +---------+- __class__          |  +---+   +---+                 
+        |         |  m_wrapped_objects -+->| *-+-->| *-+-+   B1           
+        |         +---------------------+  +---+   +---+ |  +---+        
+        |                                                +->|   |                            
+        |          ExtensionInstance                        +---+                       
+        | pb2_a1: +---------------------+   vec     InstancePtrHolder<auto_ptr<A1>,A1>
+        +---------+- __class__          |  +---+   +---+                 
+        |         |  m_wrapped_objects -+->| *-+-->| *-+-+   B2                   
+        |         +---------------------+  +---+   +---+ |  +---+        
+        |                                                +->|   | 
+        |                                                   +---+                       
+        |       ExtensionClass<A1>                                                          
+        |  A2: +------------+                                                               
+        | ()<--+- __bases__ |                                                               
+        |      |  __dict__ -+->{...}                                                        
+        |      +------------+                                                               
+        |           ^                                                                       
+        |           |  ExtensionInstance                                                    
+        |       a2: | +---------------------+   vec     InstanceValueHolder<A2>             
+        |           +-+- __class__          |  +---+   +-------------+                      
+        |           | |  m_wrapped_objects -+->| *-+-->| contains A2 |                      
+        |           | +---------------------+  +---+   +-------------+                      
+        |           |                                                                       
+        |           |  ExtensionInstance                                                    
+        |   pa2_a2: | +---------------------+   vec     InstancePtrHolder<auto_ptr<A2>,A2>  
+        |           +-+- __class__          |  +---+   +---+                                
+        |           | |  m_wrapped_objects -+->| *-+-->| *-+-+   A2                         
+        |           | +---------------------+  +---+   +---+ |  +---+                       
+        |           |                                        +->|   |                       
+        |           |  ExtensionInstance                        +---+                       
+        |   pb1_a2: | +---------------------+   vec     InstancePtrHolder<auto_ptr<A2>,A2>  
+        |           +-+- __class__          |  +---+   +---+                                
+        |           | |  m_wrapped_objects -+->| *-+-->| *-+-+   B1                         
+        |           | +---------------------+  +---+   +---+ |  +---+                       
+        |           |                                        +->|   |                       
+        |           |                                           +---+                       
+        |           |                                                                       
+        |           +---------------+------------------------------+                             
+        |                           |                              |                             
+ +------+-------------------------+-|----------------------------+ |                             
+ |                                | |                            | |                             
+ |       Class<ExtensionInstance> | |      ExtensionClass<B1>    | |      ExtensionClass<B1>     
+ | DA1: +------------+            | | B1: +------------+         | | B2: +------------+          
+(*,)<---+- __bases__ |           (*,*)<---+- __bases__ |        (*,*)<---+- __bases__ |          
+        |  __dict__ -+->{...}             |  __dict__ -+->{...}          |  __dict__ -+->{...}   
+        +------------+                    +------------+                 +------------+          
+              ^                                ^                                    ^ 
+              |        ExtensionInstance       |                                    |       
+              |  da1: +---------------------+  |    vec     InstanceValueHolder<A1,A_callback>  
+              +-------+- __class__          |  |   +---+   +---------------------+  |          
+                      |  m_wrapped_objects -+--|-->| *-+-->| contains A_callback |  |          
+                      +---------------------+  |   +---+   +---------------------+  |          
+        +--------------------------------------+                                    |
+        |  ExtensionInstance                                                        |
+    b1: | +---------------------+   vec     InstanceValueHolder<B1,B_callback>      |
+        +-+- __class__          |  +---+   +---------------------+                  |
+        | |  m_wrapped_objects -+->| *-+-->| contains B_callback |                  |
+        | +---------------------+  +---+   +---------------------+                  |
+        |                                                                           |
+        |  ExtensionInstance                                                        |
+pb1_b1: | +---------------------+   vec     InstancePtrHolder<auto_ptr<B1>,B1>      |
+        +-+- __class__          |  +---+   +---+                                    |
+        | |  m_wrapped_objects -+->| *-+-->| *-+-+   B1                             |
+        | +---------------------+  +---+   +---+ |  +---+                           |
+        |                                        +->|   |                           |
+        |  ExtensionInstance                        +---+                           |
+ pc_b1: | +---------------------+   vec     InstancePtrHolder<auto_ptr<B1>,B1>      |
+        +-+- __class__          |  +---+   +---+                                    |
+        | |  m_wrapped_objects -+->| *-+-->| *-+-+   C                              |
+        | +---------------------+  +---+   +---+ |  +---+                           |
+        |                                        +->|   |                           |
+        |                                           +---+                           |
+        |                                                                           |
+        |       Class<ExtensionInstance>    +---------------------------------------+
+        | DB1: +------------+               |  ExtensionInstance                                                    
+       (*,)<---+- __bases__ |           a2: | +---------------------+   vec     InstanceValueHolder<A2> 
+               |  __dict__ -+->{...}        +-+- __class__          |  +---+   +-------------+                      
+               +------------+                 |  m_wrapped_objects -+->| *-+-->| contains A2 |                      
+                     ^                        +---------------------+  +---+   +-------------+                      
+                     |  ExtensionInstance      
+                db1: | +---------------------+    vec     InstanceValueHolder<B1,B_callback>  
+                     +-+- __class__          |   +---+   +----------------------+ 
+                       |  m_wrapped_objects -+-->| *-+-->| contains B1_callback |              
+                       +---------------------+   +---+   +----------------------+