Removing a finite-measure subset subtracts its measure.Suggest a slogan
Let \((X,\mcm,m)\) be a measure space and let \(A,C\in\mcm\) with \(C\subseteq A\) and \(m(C) < \infty\). Then \[ m(A\setminus C) = m(A) - m(C) \] [@Fol13].
The hypothesis \(m(C)<\infty\) cannot be dropped: for Lebesgue measure with \(A = C = \RR\), the right-hand side \(\infty-\infty\) is undefined.