Theorem: MyRing.self_sub (LeanCourse/MIL/C02_Basics/solutions/Solutions_S02_Proving_Identities_in_Algebraic_Structures.lean)

State before:
R : Type u_1
inst✝ : Ring R
a : R
⊢ a - a = 0
Tactic:
rw [sub_eq_add_neg, add_right_neg]
State after:
no goals