Expanded JS source for the bookmarklets introduced at http://blog.goosoftware.co.uk/2012/04/25/twitter-bookmarklets/

twitter-bookmarklets-expanded.js

/*
 *  === Routine for toggling @replies ===
 *
 *  Step 1: Using jQuery (which Twitter already loads), select the div 
 *  elements of class stream-item that contain tweets where the 
 *  data-is-reply-to attribute is true.
 */
var tweets = $('div.tweet[data-is-reply-to=true]').parents('div.stream-item');

/*
 *  Step 2: Determine whether we're hiding or showing. If there are no replies
 *  visible then we'll show replies, otherwise we'll hide them.
 * 
 *  FAQs :)
 * 
 *  Why not just call tweets.toggle()? 
 * 
 *  Because that gets all messed up if you scroll down and Twitter auto-loads
 *  some more tweets. At that point if you've got replies hidden then the
 *  bookmarklet will hide the visible tweets and show the hidden ones.
 *  (This way the bookmarklet also plays better with the one for hiding
 *  retweets where you have retweeted replies.)
 */
var shouldShow = tweets.filter(':visible').size() == 0;

/*
 *  Step 3: Toggle the replies on or off
 */
tweets.toggle(shouldShow);

/*
 *  === Routine for toggling retweets ===
 *
 *  Exactly the same logic as for replies, only in the first line we filter
 *  div.tweet elements based on whether they have a data-retweet-id attribute.
 */
var tweets = $('div.tweet[data-retweet-id]').parents('div.stream-item');
var shouldShow = tweets.filter(':visible').size() == 0;
tweets.toggle(shouldShow);