Dinatural exercise solved: addendum

Whoops. I lifted the proof I posted yesterday out of a longer ms I'm writing. It occured to me (you know how it goes -- around 3 a.m.) that it assumed the reader already knew the dinatural transformation 1 -> (A,A). Its source is the terminal set-valued bifunctor: the constant bifunctor whose constant value is a one-element set. Its target is the "hom" bifunctor. The dinatural transformation is the one with identity maps as values. See: the reader did already know it (because, of course, it's the only dinatural from 1 to (A,A) that can be defined uniformly for all categories).