/* nat : natural numbers */
0 :: nat
s :: nat → nat

/* list: lists of natural numbers */
nil :: list
cons :: nat → list → list

map :: (nat → nat) → list → list
map(F, nil) → nil
map(F, cons(x, xs)) → cons(F(x), map(F, xs))

double :: nat -> nat
double(0) → 0
double(s(x)) → s(s(double(x)))