I'm flattered, but must point out that diff was just one of a sequence of more capable and robust versions of proof(1), which Mike Lesk contributed to Unix v3. It, in turn, copied a program written by Steve Johnson before Unix and general consciousness of software tools. Credit must also go to several people who studied and created algorithms for the "longest common subsequence" problem: Harold Stone (who invented the diff algorithm at a blackboard during a one-day visit to Bell Labs), Dan Hirschberg, Tom Szymanksi, Al Aho, and Jeff Ullman. For a legal case in which I served as an expert witness, I found several examples of diff-type programs developed in the late 1960s specifically for preparing critical editions of ancient documents. However, Steve Johnson's unpublished program from the same era appears to be the first that was inspired as a general tool, and thus as "social software". Doug