A set is a collection
of objects, called the members or elements of the set. To show that
the objects belong to a set they are put between curly brackets { }.

For example, the set of prime numbers less than 12 is given as {2,3,5,7,11},
while the set of months whose names begin with J is given as {January,
June, July}.

Sets are often labeled with a single letter, for example

F = {multiple of 5}.

The symbol ∈ is used to show that an object is a member of a set, and ∉ to
show that it is not. For example,

20 ∈ F but 21 ∉ F

When the elements of a set A are all members of a set B then A is called a subset of B,
written A ⊂ B