Theorem: MyRing.neg_neg (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
Tactic:
apply neg_eq_of_add_eq_zero
State after:
case h
R : Type u_1
inst✝ : Ring R
a : R
⊢ -a + a = 0

State before:
case h
R : Type u_1
inst✝ : Ring R
a : R
⊢ -a + a = 0
Tactic:
rw [add_left_neg]
State after:
no goals