I don't think this is a correct doctest (output should be without ">>>"). I think it may be a good idea to run the tests :) If you update code to the latest master, then probability.doctest may be run with the following command (it requires nose to be installed):

```
nltk/test/runtests.py nltk/test/probability.doctest
```
Take a look at this document for more info: https://github.com/nltk/nltk/blob/master/web/dev/local_testing.rst