proposition: Lemma issig_morphism abf a'b'f' : (morphism_sig_T abf a'b'f') <~> morphism abf a'b'f'.
proof: Proof. issig. Defined.
imports: Category.Core Functor.Core NaturalTransformation.Core Functor.Composition.Core NaturalTransformation.Paths NaturalTransformation.Composition.Core Category.Morphisms FunctorCategory.Core Pseudofunctor.Core NaturalTransformation.Composition.Laws Trunc Types.Sigma Basics.Tactics