add missing note

generators.html

@@ -284,6 +284,7 @@ rules = []
 <li>The global <code>open()</code> function opens a file and returns a file object. In this case, the file we&#8217;re opening contains the pattern strings for pluralizing nouns. The <code>with</code> statement creates what&#8217;s called a <i>context</i>: when the <code>with</code> block ends, Python will automatically close the file, even if an exception is raised inside the <code>with</code> block. You&#8217;ll learn more about <code>with</code> blocks and file objects in the <a href=files.html>Files</a> chapter.
 <li>The <code>for line in &lt;fileobject></code> idiom reads data from the open file, one line at a time, and assigns the text to the <var>line</var> variable. You&#8217;ll learn more about reading from files in the <a href=files.html>Files</a> chapter.
 <li>Each line in the file really has three values, but they&#8217;re separated by whitespace (tabs or spaces, it makes no difference). To split it out, use the <code>split()</code> string method. The first argument to the <code>split()</code> method is <code>None</code>, which means &#8220;split on any whitespace (tabs or spaces, it makes no difference).&#8221; The second argument is <code>3</code>, which means &#8220;split on whitespace 3 times, then discard the rest of the line.&#8221; A line like <code>[sxz]$ $ es</code> will be broken up into the list <code>['[sxz]$', '$', 'es']</code>, which means that <var>pattern</var> will get <code>'[sxz]$'</code>, <var>search</var> will get <code>'$'</code>, and <var>replace</var> will get <code>'es'</code>. That&#8217;s a lot of power in one little line of code.
+<li>Finally, you pass <code>pattern</code>, <code>search</code>, and <code>replace</code> to the <code>build_match_and_apply_functions()</code> function, which returns a tuple of functions. You append this tuple to the <var>rules</var> list, and <var>rules</var> ends up storing the list of match and apply functions that the <code>plural()</code> function expects.
 </ol>
 
 <p>The improvement here is that you&#8217;ve completely separated the pluralization rules into an external file, so it can be maintained separately from the code that uses it. Code is code, data is data, and life is good.