It is a common understanding about the prefix a, an. It is an Indo-European root. It negates the word it is prefixed to, makes it the opposite, but does not negate it totally. IN-constant (a-nicca), doesn't mean having no constancy. If there was no constancy, then why does my head appear in the same place every day? My head is pretty constant in its place. So the meaning is something else. The meaning is you develop the perception to look at what is inconstant, what is not-self. The most literal way of putting it in English would be "unself". That doesn't make sense, so we use instead "not-self". As you may see, NO is not a prefix, but an adverb. Pali would say, "I do not have a self," lit. "There is no self to me," if it wish to express the idea of no self.