def apply(self, content):
raise NotImplementedError<EOL>
Applies this rule to the given content. A rule can do one or more of the following:

        - Return a string; this is taken to be the transformed version of content, and will be used
          as the new content after applying this rule.
        - Modify variables (a dict). Usually, Rules that modify this dictionary will add new
          variables. However, a Rule can also delete or update key/value pairs in the dictionary.