case hnil.hsingle =>
  simp only [List.nil_append, List.disj'_singleton, List.disj'_nil];
  apply iff_intro!;
  . simp;
  . exact disj₃''! efq! imp_id!;
F : Type u_1
inst✝⁵ : LogicalConnective F
inst✝⁴ : DecidableEq F
S : Type u_2
inst✝³ : System F S
𝓢 : S
inst✝² : Minimal 𝓢
p q r : F
inst✝¹ : System.NegationEquiv 𝓢
inst✝ : HasEFQ 𝓢
a✝ : F
⊢ 𝓢 ⊢! ([] ++ [a✝]).disj' ⟷ [].disj' ⋎ [a✝].disj'
no goals