Compare two FENs and return a move?

[mixographer]

I'm wondering if anyone has any examples of comparing two FENs and returning the move in a from-to notation like

"rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
"rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1"

returns e2e4.

C, C++, or python would be great, but any help would be a great help. Handling castle - cases where there are two changes in the FEN position would be great too.

Thanks.