Travis explains:

we've separated what to diff with how to diff completely. This means that I can have a method on RBScanner which scans source tokens. But it can go further. Since said scanner knows where comments are, we can produce strings for the comments. In fact we can go further, we can break the comments up into words easily. We can even note when the token is a string literal token and break it up into words as well. And we can insert elements for white space. So that when we difference Smalltalk methods, we get everything broken down to the granularity we'd like.