o run DocTest test cases, and accumulate statistics.
    The `run` method is used to process a single DocTest case.  It
    returns a tuple `(f, t)`, where `t` is the number of test cases
    tried, and `f` is the number of test cases that failed.

        >>> tests = DocTestFinder().find(_TestClass)
        >>> runner = DocTestRunner(verbose=False)
        >>> tests.sort(key = lambda test: test.name)
        >>> for test in tests:
        ...     print(test.name, '->', runner.run(test))
        _TestClass -> TestResults(failed=0, attempted=2)
        _TestClass.__init__ -> TestResults(failed=0, attempted=2)
        _TestClass.get -> TestResults(failed=0, attempted=2)
        _TestClass.square -> TestResults(failed=0, attempted=1)

    The `summarize` method prints a summary of all the test cases that
    have been run by the runner, and returns an aggregated `(f, t)`
    tuple:

        >>> runner.summarize(verbose=1)
        4 items passed all tests:
           2 tests in _TestClass
           2 tests in _TestClass.__init__
           2 tests in _TestClass.get
           1 tests in _TestClass.square
        7 tests in 4 items.
        7 passed and 0 failed.
        Test passed.
        TestResults(failed=0, attempted=7)

    The aggregated number of tried examples and failed examples is
    also available via the `tries` and `failures` attributes:

        >>> runner.tries
        7
        >>> runner.failures
        0

    The comparison between expected outputs and actual outputs is done
    by an `OutputChecker`.  This comparison may be customized with a
    number of option flags; see the documentation for `testmod` for
    more information.  If the option flags are insufficient, then the
    comparison may also be customized by passing a subclass of
    `OutputChecker` to the constructor.

    The test runner's display output can be controlled in two ways.
    First, an output function (`out) can be passed to
    `TestRunner.run`; this function will be called with strings that
    should be displayed.  It defaults to `sys.stdout.write`.  If
    capturing the output is not sufficient, then the display output
    can be also customized by subclassing DocTestRunner, and
    overriding the methods `report_start`, `report_success`,
    `report_unexpected_exception`, and `report_failure`.
    zF**********************************************************************Nr"