Subset
A subset, a collection of parts of a collection, is also called a collection of parts. If X and Y are sets, and all elements of X are elements of Y, there are: ▪ X is a subset of Y (or included in Y); ▪ X ⊆ Y; ▪ Y is the parent of x (Including X); ▪ Y ⊇ X. All sets Y are their own subsets. A subset of Y that is not equal to Y is called a true subset. If X is a true subset of Y, then write X ⊊ Y. "Is a subset of ..." is called inclusion. ...