Theorem: MyRing.add_neg_cancel_right (LeanInRome/S2_Basics/S02_Proving_Identities_in_Algebraic_Structures.lean)

State before:
R : Type u_1
inst✝ : Ring R
a b : R
⊢ a + b + -b = a
Tactic:
sorry
State after:
no goals