Ok, so I was reading the Wikipedia article on Yonedas lemma. And I've heard before that when you prove things in category theory you automatically get a lot of results by proving it in abstract categories. 

One of my favorite things in math is the whole Lagrange's theorem $\rightarrow$ Euler's theorem $\rightarrow$ Fermat's little theorem chain.

Are there examples of other "semi-mainstream" theorems trivialized by Yoneda?

Regards