= \{u\mid u = t\}\), for each \(t\) a singleton set \(\{t\}\) can be constructed that contains \(t\) as its only element. Note that \(t\in\{t\}\) union : \(X\cup Y = \{t\mid t \in X \vee t \in Y\}\) (\(\vee\) means "or") intersection : \(X\cap Y = \{t\mid t\in X \wedge t\in