Theorem: MyRing.eq_neg_of_add_eq_zero (LeanCourse/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:
sorry
State after:
no goals