module Fib;

type Nat :=
  zero : Nat
  | suc : Nat → Nat;

infixl 6 +;
+ : Nat → Nat → Nat;
+ zero b := b;
+ (suc a) b := suc (a + b);

fib : Nat -> Nat;
fib zero := zero;
fib (suc (zero)) := suc zero;
fib (suc (suc n)) := fib (suc n) + fib n;

end;