``compatibility_alias``

The keyword ``compatibility_alias`` allows you to define a class name
as equivalent to another class name.  For example:

.. code-block:: c++

  compatibility_alias WOApplication GSWApplication;

tells the compiler that each time it encounters ``WOApplication`` as
a class name, it should replace it with ``GSWApplication`` (that is,
``WOApplication`` is just an alias for ``GSWApplication``).

There are some constraints on how this can be used

* ``WOApplication`` (the alias) must not be an existing class;

  * ``GSWApplication`` (the real class) must be an existing class.

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