Theorem: MyRing.neg_eq_of_add_eq_zeroαα (MIL/C02_Basics/S02_Proving_Identities_in_Algebraic_Structures.lean)

State before:
R : Type u_1
inst✝ : Ring R
a b : R
h : a + b = 0
⊢ -a = b
Tactic:
rw [← neg_add_cancel_left a b, h, add_zero]
State after:
no goals