fact: Definition Under_precategories_mor_mor {c c' : C} (h : C⟦c, c'⟧) (af af' : c' / C) (g : (c' / C)⟦af, af'⟧) : (c / C) ⟦Under_precategories_mor_ob h af, Under_precategories_mor_ob h af'⟧.
imports: UniMath.Foundations.PartD UniMath.Foundations.Propositions UniMath.Foundations.Sets UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Functors