proposition: Proposition two_of_six_is_prop {C : category} (W : morph_class C) : isaprop (two_of_six W).
proof: Proof. unfold two_of_six. repeat (apply impred ; intro). apply isofhleveltotal2. apply (W _ _ _). intro. apply isofhleveltotal2. apply (W _ _ _). intro. apply isofhleveltotal2. apply (W _ _ _). intro. apply (W _ _ _). Defined.
imports: UniMath.Foundations.All UniMath.MoreFoundations.All UniMath.CategoryTheory.Monics UniMath.CategoryTheory.Epis UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Functors UniMath.CategoryTheory.Core.Isos