Owen wrote:

> Puzzle 124: I don't think there's just one functor from \$$\mathcal{D}\$$ to \$$\mathcal{C}\$$. There is just one graph homomorphism, though.

You're right!

I'll fix the puzzle by indicating the functor I'm actually interested in: the one that maps the morphisms \$$\textrm{FriendOf}\$$ and \$$\textrm{FriendOf}^\prime\$$ in \$$\mathcal{D}\$$ to the morphism \$$\textrm{FriendOf}\$$ in \$$\mathcal{C}\$$.

> (Feel free to delete this paragraph after reading it.)

I'll leave it in! I don't want to convey the false impression that I'm infallible. I do, however, want to fix up my "lectures" so that students don't get confused by mistakes.

Anindya wrote:

> Could we not send one or both FriendOf in \$$\mathcal{D}\$$ to the identity map in \$$\mathcal{C}\$$?

Yes, that's one of infinitely many other functors from \$$\mathcal{D}\$$ to \$$\mathcal{C}\$$ I hadn't noticed when first stating Puzzle 124.