change a + b + 0 = a + (b + 0)
case zero
S : Type
image_zero : S
image_succ : N → S → S
n m a b c : N
⊢ a + b + zero = a + (b + zero)
case zero
S : Type
image_zero : S
image_succ : N → S → S
n m a b c : N
⊢ a + b + 0 = a + (b + 0)