That’s it! I’ve finally hit on it—the formula that WILL get through to people who persist in writing the likes of “I’m going to sign my niece and I up for lessons.”
UPDATE: A friend on Facebook responded, “Huh?”
I wanted to say, “If you have to ask, you’ll never know.”
And this is someone who studied Latin in high school!
I’ll put my reluctant explanation in the comments.
The Compulsive Copyeditor 
