Function NTH

{tt nth} returns the item in the list specified as its first
argument in the position specified as its second argument.
Arity: 3
Axioms:
(Nth-Domain Nth 2 Natural)

(Nth-Domain Nth 1 List)

(=> (Nth ?List ?N ?Result)
    (= ?Result
       (Cond ((= ?N 1) (First ?List))
             ((Positive ?N) (Nth (Rest ?List) (1- ?N))))))