Except, as the slides indicate, sequences of values are not sets. They are ordered. You can also, as they mention, refer to "y" in the definition of "x". This i

Except, as the slides indicate, sequences of values are not sets. They are ordered. You can also, as they mention, refer to "y" in the definition of "x". This is like set union, but it is, again, sensitive to ordering of elements.