I know a number of people have suggested ending this thread, and I think it's probably time to, but the following comment seems to me to belong here:

I too was bemused by YART. I never tried to look it up. I just worked round it till I figured out the meaning from context. And then decided, for myself, that it stood for 'You already read this.'

I was wrong! But I am nostalgic now for that simple little sentence I was so proud of myself for working out...