Substitutes the elements of <lis1> at position(s) <posn> (if they exist) with the corresponding elements in <val>. Ex. (subs-posn '(0 1 2 3) 2 'a) => (0 1 a 3) Ex. (subs-posn '(0 1 2 3) '(1 3) '(a b)) => (0 a 2 b)