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 : \(X\cap Y = \{t\mid t\in X \wedge t\in Y\}\) complement : \(\complement X = \{t \mid t \notin X\}\) (unordered) pair : \(\{t,u\} =