University of California, San Diego: Edward Bender and S. Williamson's "Sets, Equivalence and Order: Sets and Functions"
Study Theorem 1 on pages SF-2 and SF-3. Double complement is also referred to as double negative: (A') ' = A. Take the identities in the theorem involving set difference or complement. Insert a second difference or complement operator, thus creating some new set equations, and check whether the new set equations are still identities. Prove or disprove several of the equations you created by adding a second difference or complement operation.