-- Peano arithmetic

a + 0 = a
0 + b = b
(S a) + b = S (a + b)
a + (S b) = S (a + b)

a * 0 = 0
0 * b = 0
(S a) * b = a * b + b
a * (S b) = a * b + a

Nat 0 = 0
Nat n = S (Nat (n - 1))

Main = (Nat 3) * (Nat 7)