Definition 6.1.3 (Slice).label Let $X, Y$ be sets, $U \subset X \times Y$, and $A \subset X$, then
\[U(A) = \bracs{y \in Y: (x, y) \in U, x \in A}\]
is the slice of $U$ at $A$.
Definition 6.1.3 (Slice).label Let $X, Y$ be sets, $U \subset X \times Y$, and $A \subset X$, then
is the slice of $U$ at $A$.