3 + 3
     : Z
3 + 3
     : znat
-4
     : Z
Init.Nat.add
     : nat -> nat -> nat
S
     : nat -> nat
Init.Nat.mul
     : nat -> nat -> nat
le
     : nat -> nat -> Prop
plus
     : nat -> nat -> nat
succ
     : nat -> nat
Init.Nat.mul
     : nat -> nat -> nat
le
     : nat -> nat -> Prop
