9・4 Find all pairs of numbers $(x, y)$ that satisfy
$$|\sin x-\sin y|+\sin x \cdot \sin y \leqslant 0$$
[Solution] If $(x, y)$ satisfies the given inequality, then $\sin x \cdot \sin y \leqslant 0$,
from which we can get $|\sin x-\sin y|=|\sin x|+|\sin y|$.
Thus, $(x, y)$ satisfies $|\sin x|+|\sin y|+\sin x \cdot \sin y \leqslant 0$, hence $\sin x=\sin y=0$.
The set of all pairs $(x, y)$ is
$$\{(x, y) \mid x=n \pi, y=m \pi, n, m \in \mathbb{Z}\} .$$