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I agree that users should be able to override the template.

I think Raymond was concerned about 

(1) backwards compatibility if you want to change the 
interface.  For instance, you might later decide that there 
should be separate templates for header/footer/match section/
changed line/added line/deleted line.

(2) matching the other diff options, so this doesn't look far 
more complicated.

Unfortunately, at the moment I don't see a good way to solve 
those; using a private member and access functions wouldn't 
really simplify things much.