"Elves are very good at board games, and I'm NOT an elf!"
Here is the Officer White card. There is only 1 Officer White card in the entire deck.
Officer White can be played on any players doubled meld to reverse the white card's doubling action. In this way a player who has managed to, for instance, lay a meld worth 30 and then has doubled it (using a white evidence card), making the meld worth 60, can have Officer White played to the meld by another player to make the meld worth 30 again.