IF YOU CAN SEE THIS go to http://www.cs.cornell.edu/Info/People/sfa/Nuprl/Shared/Xindentation_hack_doc.html
At:
general length nth tl r:, L:Top List. ||nth_tl(r;L)|| = if r<||L|| ||L||-r else 0 fi
By:
InductionOnNat THEN RecUnfold `nth_tl` 0 THEN Reduce 0