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doctest example
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You can create documents with code examples. They look like prompt extracts.
Python provides a module called `doctest` that knows how to extract code
examples out of a text file, to execute them.

For example, this piece of code::

    >>> 1 + 1
    2

Can be interpreted by `doctest`, to ensure that the result is 2.

When an example fails, `doctest` raises an assertion error, like 
a classical unit test would do::

    >>> 1 + 1
    2