================================================================
The Hook Pattern
================================================================

For calls originating from system callbacks and/or data structures,
such frame tree messages and draw tree messages, we must hook calls
out and back again in order to provide an opportunity for overriding.

The Frame class will be used as an example.

_______________________________________________________________________

Inheritance diagram:


               Frame
                 |
                 |
                 |
                 |
                 |
                 |
            impl::Frame



            Frame_Stowed
                 |
                 |
                 |
                 |
                 |
         impl::Frame_Stowed

_______________________________________________________________________

Containment/call diagram:

impl::Frame_Stowed contains an instance of Own_Frame and an instance
of Own_Frame_Hook.

        +-----------------------------------+
        |                                   |
        | class: Frame_Stowed               |
        |                                   |
        | Frame & peek_frame() = 0          |
        |                                   |
        +===================================+
        |                                   |
        | derived class: impl::Frame_Stowed |
        |                                   |
   +------Frame & peek_frame()              |
   |    |    *                              |
   |    +---/|\-----------------------------+
   |         |            
   |         |
   |         |peek_frame().foo()
   |         |
   |         |
   |         |              +---------------------------------+
   |         |              |                                 |
   |         |              | class: Frame                    |
   |         |              |                                 |
   |         |              | void foo() = 0                  |
   |         |              |                                 |
   |         |              +=================================+
   |         |              |                                 |
   |         |              | derived class: Own_Frame_Hook   |
   |         |              |                                 |
   |         |       +--------void foo()                      |
   |         |       |      |       *                         |
   |         |       |      +------/|\------------------------+
   |         |       |              |        
   |         |       |              |        
   +--------------------------------+        
             |       |  
             |       |  
             |       |      +---------------------------------+
             |       |      |                                 |
             |       |      | class: impl::Frame              |
             |       |      |                                 |
             |       +------->void foo()                      |
             |              |                                 |
             |              +=================================+
             |              |                                 |
             |              | derived class: Own_Frame        |
             |              |                                 |
             +----------------void foo()                      |
                            |       *                         |
                            +------/|\------------------------+
                                    |
                                    |
                                    |
                                    |foo()
                                    |
                                    |
                                    |
                          (START: parent frame)