rw [List.eq_nil_iff_forall_not_mem]
C : Type u
τ : outParam (Type v)
inst✝⁴ : Insert C τ
inst✝³ : Membership τ C
inst✝² : Mem C τ
inst✝¹ : ToList C τ
inst✝ : Mem.ToList C τ
⊢ toList empty = []
C : Type u
τ : outParam (Type v)
inst✝⁴ : Insert C τ
inst✝³ : Membership τ C
inst✝² : Mem C τ
inst✝¹ : ToList C τ
inst✝ : Mem.ToList C τ
⊢ ∀ (a : τ), a ∉ toList empty