Theorem: Tutorial.mul_inv_eq_one (Tutorial/Advanced/Algebra/Lecture1.lean)

State before:
A : Type ?u.3960
G : Type
inst✝¹ : Ring A
inst✝ : Group G
a b : G
⊢ a * b⁻¹ = 1 ↔ a = b
Tactic:
sorry
State after:
no goals