grammar
T {{ hol Typ }}, S, U :: Ty ::=                     {{ com types: }}
  | Nat                          ::   :: Nat          {{ com type of natural numbers }}

defns
Jtype :: '' ::= 

defn
G |- t : T :: :: typing :: T_ {{ com Typing }} by

------------------ :: Zero
G |- 0 : Nat

G |- t1 : Nat
------------------ :: Succ
G |- succ t1 : Nat

G |- t1 : Nat
------------------ :: Pred
G |- pred t1 : Nat

G |- t1 : Nat
--------------------- :: IsZero
G |- iszero t1 : Bool
