>>> from enthought.traits.api import HasTraits, This
>>> class Employee(HasTraits):
...    manager = This
...
>>> class Executive(Employee):
...  pass 
...
>>> fred = Employee()
>>> mary = Executive()
>>> fred.manager = mary 
>>> # This is OK, because fred's manager can be an instance of
>>> # Employee or any subclass.
>>> mary.manager = fred 
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\wrk\src\lib\enthought\traits\trait_handlers.py", line 
90, in error
    raise TraitError, ( object, name, self.info(), value )
enthought.traits.trait_errors.TraitError: The 'manager' trait of 
an Executive instance must be an instance of the same type as the 
receiver, but a value of <__main__.Employee object at 0x00994330> 
was specified.
