Its interesting that despite the utility of S-Expressions, as mentioned, semantic diff, for example, of CL code is uncommon.

By that I mean highlighting the diff between these:

  (dolist (i l)
    (print (car i)))

  (dolist (i l) (print (cdr i)))

I'm pretty sure this exists, but it's uncommon (at least to me its uncommon).

It is uncommon because it turns out that text diff is good enough 99% of the time, especially if you follow normal formatting and indentation conventions.

Also, structural diff is actually a very hard problem.