Multiset overlap ($|A\cap B|={\sum }_x\min (m_A(x),m_B(x))$)

The common multiplicity of two multisets, summing the smaller multiplicity of each value. To return an order-preserving overlap of two finite lists, scan the first list and consume one equal item from the second whenever available. Each token can be consumed at most once, so the resulting subsequence has maximum possible length among those that occur in a permutation of the second list.