foldl
F
x
nil
x
foldl
F
x
cons
y
l
foldl
F
F
x
y
l
plusc
x
nat
y
nat
plus
x
y
sum
l
foldl
plusc
0
l
x
nat
y
nat
F
nat
nat
nat
l
list
0
nat
plus
nat
nat
nat
nil
list
cons
nat
list
list
foldl
nat
nat
nat
nat
list
nat
sum
list
nat
plusc
nat
nat
nat
FULL
.