You made me chuckle . . . When i learned chess in 1952 you were expected to say " en Guarde" when the opposing Queen was attacked and could be taken the next move. . .
BUT that was when it was a gentleman's game . . .
Now it's expected that the opponent SHOULD be alert . . .
Hm. Nobody has said that to me in 70 years . . .
It's not something that I invented, but have only played against 2 people that I didn't tell them what it was, that used it.