A set is a collection of things (called elements), without regard to their order. An item is either in a set (it is an element of the set), or it is not. It cannot be in the set more than once. Two sets are equal if they contain electly the same elements. For instance, the set {1, 2, 3, 4} is equal to the set {1, 4, 3, 2}, but not to the set {0, 1, 2, 3}. As another example, the set {1, 2, 2} is equal to the set {1, 2}: they have the same two (distinct) elements, 1 and 2.