A planar vector field $\mathbf{v}(x, y)=(u(x, y), v(x, y))^T$ is called irrotational if it has zero divergence: $\nabla \cdot \mathbf{v}=\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y} \equiv 0$. Prove that the set of all irrotational vector fields is a subspace of the space of all planar vector fields.