Explore the theorem of Inclusion/Exclusion in Set Theory through an unorthodox lens focusing on multisets rather than traditional sets in this 48-minute video lecture. Delve into a unique approach that reduces the theorem to its arithmetic counterpart and apply it to the Euler phi function. Expand your understanding of multisets to include integral_msets, where element multiplicities can be any integer, not just natural numbers or zero. Discover how this broader perspective allows for differences between multisets, providing the flexibility needed for a conceptual and general statement of the Inclusion/Exclusion theorem. Learn about multiset operations such as sums, unions, intersections, and differences, and gain insights into the modern interpretation of negative numbers in counting exercises.