       assert CustomSet.equal?(actual, expected)
     end
 
+    @tag :pending
     test "difference of a non-empty set and an empty set is the non-empty set" do
       custom_set_1 = CustomSet.new([1, 2, 3, 4])
       custom_set_2 = CustomSet.new([])

      assert CustomSet.equal?(actual, expected)
    end

    test "difference of two non-empty sets is a set of elements that are only in the first set" do
      custom_set_1 = CustomSet.new([3, 2, 1])
      custom_set_2 = CustomSet.new([2, 4])

      assert CustomSet.equal?(actual, expected)
    end

    @tag :pending
    test "union of non-empty sets contains all unique elements" do
      custom_set_1 = CustomSet.new([1, 3])
      custom_set_2 = CustomSet.new([2, 3])
