4.2.2 The type constraint of a slot

The type constraint of the total-seconds slot is <integer>. This slot can hold instances of <integer>, and cannot hold any other kind of object.

The type constraint of a slot is optional. Specifying a slot with no explicit type constraint is equivalent to specifying <object> as the type constraint. A slot whose type constraint is <object> can hold any object. The ability to have slots with the type constraint <object> provides flexibility that can be valuable; for more information, see Chapter 19, Performance and Flexibility.