nul
0
true
nul
s
X
false
nul
err
false
pred
0
err
pred
s
X
X
id
X
X
eq
0
nul
eq
s
X
z
nat
eq
X
pred
z
add
0
id
add
s
X
z
nat
add
X
s
z
X
nat
Y
nat
0
nat
s
nat
nat
err
nat
pred
nat
nat
id
nat
nat
add
nat
nat
nat
nul
nat
bool
eq
nat
nat
bool
true
bool
false
bool
FULL
.