  The first primary expression given was for All \(X\) is \(Y\), an equation which he then converted into \(x(1-y) = 0\). This was the first appearance of 0 in MAL. It was not introduced as the symbol for the empty class—indeed the empty class does not appear in MAL. Evidently “\(= 0\)” performed the role of a predicate in MAL, with an equation \(E = 0\) asserting that the class denoted by \(E\) simply did not exist. (In LT, what we call the empty class was introduced and denoted by 0.) 