\begin{definition}[Definition:Class Difference]
Let $A$ and $B$ be two classes.
The '''(class) difference''' $A \setminus B$ of $A$ and $B$ is defined as the class of all sets $x$ such that $x \in A$ and $x \notin B$:
:$x \in A \setminus B \iff x \in A \land x \notin A$
or:
:$A \setminus B = \set {x: x \in A \land x \notin B}$
\end{definition}