fact: Definition associator_to_nat_trans (α' : associator): nat_trans assoc_left assoc_right := nat_z_iso_to_trans α'.
imports: UniMath.Foundations.PartD UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Isos UniMath.CategoryTheory.Core.NaturalTransformations UniMath.CategoryTheory.Core.Functors UniMath.CategoryTheory.ProductCategory UniMath.CategoryTheory.PrecategoryBinProduct UniMath.CategoryTheory.whiskering