A cons is a compound object having two components, called the car and cdr. These form a dotted pair. Each component can be any object.
(cons [car-typespec [cdr-typespec]])
car-typespec—a type specifier, or the symbol *. The default is the symbol *.
cdr-typespec—a type specifier, or the symbol *. The default is the symbol *.
This denotes the set of conses
whose car is constrained to be of type car-typespec and
whose cdr is constrained to be of type cdr-typespec.
(If either car-typespec or cdr-typespec is *,
it is as if the type
t had been denoted.)
Section 2.4.1 (Left-Parenthesis), Section 18.104.22.168 (Printing Lists and Conses)