Let $M = (X, \mathcal{F})$ be a D1700: Discrete measurable space such that

(i)	$\mu : \mathcal{F} \to [0, \infty]$ is a D4105: Standard counting measure on $M$
(ii)	$I_X : X \to \{ 0, 1 \}$ is an D41: Indicator function on $M$ with respect to $X$

Then \begin{equation} \forall \, E \in \mathcal{F} : \mu(E) = \sum_{x \in E} I_X(x) \end{equation}