Mistake in the Orthography Rules


Παλαιό Μέλος
When I was collecting rules of orthography, I found a rule that stated: "Kentēmata with a gorgón must be changed into an oligon with a gorgón when followed by a petastē." Assuming it was correct, I included it in my collection of orthography rules. But now I noticed that Gregorios the Protopsaltis, Theodoros Phokaeus (and perhaps other classical composers) did not follow this rule. Therefore, I conclude that the person who wrote this rule was mistaken. (Unfortunately, I don't remember now which book had this rule.)