intro c hi a b
case hs

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