IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html
At:
select nth tl21 1. T : Type
2. T List
3. u : T 4. v : T List
5. n:{0...||v||}, i:(||v||-n). nth_tl(n;v)[i] = v[(i+n)]
6. n : {0...||v||+1}
7. i : (||v||+1-n)
8. n0
(u.v)[i] = (u.v)[(i+n)]