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

State before:
R : Type u_1
inst✝ : Ring R
a b : R
⊢ -a + (a + b) = b
Tactic:
rw [← add_assoc, add_left_neg, zero_add]
State after:
no goals