∀ {Γ : Type u_1} {R : Type u_3} [inst : PartialOrder Γ] [inst_1 : AddMonoid R] (x : HahnSeries Γ Rᵃᵒᵖ),
  (AddOpposite.unop (HahnSeries.addOppositeEquiv x)).leadingCoeff = AddOpposite.unop x.leadingCoeff