I was under the impression law is essentially "append only". New laws override existing laws, but the text of the existing law never changes.
See: http://en.wikipedia.org/wiki/United_States_Code#Legal_status
Someone needs to take the plunge and start writing the program; throw it on Github and tell us all about it. I know people who are looking for such a tool.
"When sections are repealed, their text is deleted and replaced by a note summarizing what used to be there." https://en.wikipedia.org/wiki/United_States_Code#Treatment_o...
"Remove pages 123.4 - 123.6 and replace with pages 123.4a-123.7."
Later, when I learned about diffs, I understood the concept immediately.