pair = \ x y z . z x y;

first = \ p . p (\ x y . x);
second = \ p . p (\ x y . y);

cons = pair;
head = first;
tail = second;
nil = \ x y . y;
isNil = \ l . l (\ h t d . (\ x y . y)) (\ x y . x);

main = first (pair a b);