nil :: List
cons :: Int -> List -> List

gcd :: Int -> Int -> Int
gcd(n, m) -> gcd(-n, m)    | n < 0
gcd(n, m) -> gcd(n, -m)    | m < 0
gcd(n, 0) -> n
gcd(n, m) -> gcd(m, n % m) | n >= 0 /\ m > 0

fold :: (Int -> Int -> Int) -> Int -> List -> Int
fold(F, n, nil) -> n
fold(F, n, cons(x, y)) -> F(x, fold(F, n, y))

gcdlist :: List -> Int
gcdlist(l) -> fold(gcd, 0, l)