Pattern defines that an object of a class of this type can have only one
instance active at any time and that it provides a Global point of access to it
from a well known access point.
office of the President of the United
States is a Singleton. The United States
Constitution specifies the means by which a president is elected, limits the
term of office, and defines the order of succession. As a result, there can be
at most one active president at any given time. Regardless of the personal identity
of the active president, the title, "The President of the United States"
is a global point of access that identifies the person in the office.