There's clearly a notation for isomorphism. It's just $\cong$. But is there notation to indicate that an isomorphism is natural? And in general, for morphisms which are natural?

I ask because one of my professors complained about the fact that mathematicians haven't developed a notation, and I may remember hearing of such a notation, but I can't remember what it was.

whatthe isomorphism is (or perhaps its inverse), or at least how it can be uniquely characterized (so it can be used in many ways). – BCnrd Nov 11 '10 at 5:03