. intro h
  conv at h =>
    rhs
    rw [← Nat.zero_add b]
  exact Nat.add_right_cancel h
case mp
a b : Nat
⊢ a + b = b → a = 0

case mpr
a b : Nat
⊢ a = 0 → a + b = b
case mpr
a b : Nat
⊢ a = 0 → a + b = b