There are predictable alternations, and are also unpredictable alternations. The rule that formulates unpredictable alternation isn`t predictable, also isn`t proper, yet we cannot say that the rule isn`t rule if according to definition of rule. Because a rule is defined, whatever it is, as anything that formulates alternation. That rule is of course just non-proper on the view of veridity of rule. It is merely a problem of interpretation that a relation of two different surface forms [x] and [y] are alternation or the former and latter form of change. Regardless of if The phenomenon applied in [x] and [y] is interpreted in alternation or change, it is obvious fact that some phonological phenomenon had been applied between [x] and [y]. Some people severely have restricted synchronic alternation only to conjugation and declension. But if it does so, we cannot suppose synchronic alternation into word. A word can have one more than allomorph. And there isn`t any evidence that a phonological phenomena appeared into word is diachronic one. Underlying form is just a existence interpreted. So we have to decide whether any alternation is synchronic one or diachronic one according to property of phenomenon itself.