\begin{definition}[Definition:Doubleton/Class Theory]
Let $a$ and $b$ be sets.
The class $\set {a, b}$ is a '''doubleton (class)'''.
It is defined as the class of all $x$ such that $x = a$ or $x = b$:
:$\set {a, b} = \set {x: x = a \lor x = b: a \ne b}$
\end{definition}