> [!NOTE] Lemma > Let $A$ be a finite set. Let $(A_{i})_{i=1}^{n}$ be a list of pairwise disjoint subsets of some set then $\left\lvert \bigcup_{i=1}^{n} A_{i} \right\rvert =\sum_{i=1}^{n} \lvert A_{i} \rvert $