inductive empty : Set \def .
-let rec pos2nat (x:pos) : nat \def
+let rec pos2nat x \def
match x with
[ one \Rightarrow (S O)
| (next z) \Rightarrow S (pos2nat z)].
-let rec nat2int (x:nat) :int \def
+let rec nat2int x \def
match x with
[ O \Rightarrow positive O
| (S z) \Rightarrow positive (S z)].
coercion nat2int.
-let rec plus x y : int \def
+let rec plus x y \def
match x with
[ (positive n) \Rightarrow x
| (negative z) \Rightarrow y].