A list is a chain of conses in which the car of each
cons is an element of the list, and the cdr of
each cons is either the next link in the chain or a terminating
A proper list is a chain of conses terminated by
the empty list, (), which is itself a proper list.
A dotted list is a list which has a terminating atom
that is not the empty list.
A circular list is a chain of conses that has no termination
because some cons in the chain is the cdr of a later cons.
Dotted lists and circular lists are also lists, but usually
the unqualified term "list" within this specification means proper list.
Nevertheless, the type list unambiguously includes dotted lists
and circular lists.
For each element of a list there is a cons.
The empty list has no elements and is not a cons.
The types cons and null form an exhaustive partition
of the type list.