cons cell

    /konz sel/ or /kons sel/ A Lisp pair object
   containing any two objects.  In Lisp, "cons" (short for
   "construct") is the fundamental operation for building structures
   such as lists and other binary trees.  The application of
   "cons" to objects H and T is written

   	(cons H T)

   and returns a pair object known as a "cons", "cons cell" or
   dotted pair.

   Typically, a cons would be stored in memory as a two consecutive
   pointers.

   The two objects in a cons, and the functions to extract them, are
   called "car" and "cdr" after two 15-bit fields of the machine
   code instruction format of the IBM 7090 that hosted the
   original LISP implementation.  These fields were called the
   "address" and "decrement" parts so "car" stood for "Contents of
   Address part of Register" and "cdr" for "Contents of Decrement
   part of Register".

   In the typical case where the cons holds one node of a list
   structure, the car is the head of the list (first element) and
   the cdr is the tail of the list (the rest).  If the list had
   only one element then the tail would be an empty list, represented
   by the cdr containing the special value "nil".

   To aid in working with nested structures such as lists of lists,
   Lisp provides functions to access the car of the car ("caar"), the
   car of the cdr ("cadr"), the cdr of the car ("cdar") and the cdr
   of the cdr ("cddr").

   (2014-11-09)