Here is another problem from Bondy/Murty: Prove that
A directed graph admits a decomposition into directed cycles if and only if it is even.
Here a directed graph is even if all its vertices have the same in- and out-degree. This is the digraph-version of Veblen's theorem which is proved by induction in the book. I don't see how I can "convert" that proof into a proof for digraphs, and I can't come up with anything else.