specialize hi a
case hs
b : ↑N0
hi : ∀ (a : ↑N0), plus (S a, b) = S (plus (a, b))
a : ↑N0
⊢ plus (S a, S b) = S (plus (a, S b))
case hs
b a : ↑N0
hi : plus (S a, b) = S (plus (a, b))
⊢ plus (S a, S b) = S (plus (a, S b))