`__call__` chaining

call_chain.txt

>>> class Foo:
...     def __call__(self):
...         return 42
...         
>>> class Bar:
...     __call__ = Foo()
...     
>>> class Baz:
...     __call__ = Bar()
...     
>>> class Spam:
...     __call__ = Baz()
...     
>>> spam_instance = Spam()
>>> spam_instance()
42