Even Greener Pastures

I can't really prove it right now, but yes, I seem to remember it was mentioned in one of the Cambridge books I had to use in class. Plus, if I hadn't had to teach it, I don't think I'd have been able to come up with the rule at the drop of a hat. I'm not surprised you can't remember being taught about this in school, though. As you said, you use whatever's easiest on the tongue. It's just that this fits nicely into a rule (for once).