There are two types of relationships between types.
Subtype/Supertype relationship:-
If x is a supertype of y, then any object of type y is also of type x. And y is said to be a subtype of x.
The type t is a supertype of every type whatsoever. Then every object is of type t.
The type nil is a subtype of every type whatsoever. No object is of type nil.
Example: In LISP the type simple-array is a subtype of array.
Disjointness of Types:-
If types x and y are disjoint, then in any implementation no object may be both of type x and of type y.
Example: If type x contains (1 … 50) and type y contains (100 ….. 150) objects. There is no object which exists in both of type x and type y. Then type x and type y are disjoint types.
In LISP the types integer and ratio are disjoint subtypes of rational.
