Installing Parsers/Tree Builders to, and accessing these packages from Python2.7

[Simon Evans]

Dear Mark Lawrence, 
I have tried inputting the code in the first link, re:
>>> import lxml
>>> import lxml.etree
>>> import bs4.builder.htmlparser
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named htmlparser
>>> import bs4.builder._lxml
>>> import bs4.builder.html5lib
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named html5lib
>>>

which tells me lxml is installed, but that neither html nor html5 is installed.