Let $\chi_S(x)$ denote the characteristic function of a set $S$ (where $\chi_S(x) = 1$ if $x \in S$ and $0$ if $x \notin S$). If $A$ and $B$ are subsets of a universal set $U$, which set $Y$ is defined by the characteristic function $\chi_Y(x) = \chi_A(x) \cdot (1 - \chi_B(x))$?