What $+$ and $-$ after $\nabla$ mean? For example, from NMF with the beta-divergence,
"Let $θ$ be a coefficient of W or H. As will be seen later, when using the β-divergence the derivative $\nabla_θ{D(θ)}$ of the criterion $D(V|WH)$ with respect to $θ$ can be expressed as the difference of two nonnegative functions such that $$\nabla_θ{D(θ)} = \nabla_θ^+{D(θ)} - \nabla_θ^-{D(θ)}$$
May you provide easy example regarding this?