articles/lisp.md: add cons cells drawing

articles/lisp.md

@@ -14,6 +14,49 @@ time. Therefore, Lisp source code directly represents the program as a tree with
 A PDF of the original Lisp 1.5 manual can be found here: <https://www.lispmachine.net/books/LISP_1.5_Programmers_Manual.pdf>. Head to page 13 for
 enlightenment.
 
+## Cons cells and Lists
+A list in Lisp is composed of `cons` cells which have two fields: the contained value of the cell known as `car` and a pointer to the
+next cell known as `cdr`. To mark the end of a list, a special sentinel value, `NIL` is used, which represents an empty list.
+
+```
+A cons cell:
+
+    +---+---+    
+    | O | O-|----> cdr
+    +---+---+    
+      |          
+     `'´         
+     car
+
+To create a cell, `cons` is used:
+    (cons 1 2)       ; 1 and 2 are the car and cdr respectively
+    (car (cons 1 2)) ; ==> 1
+    (cdr (cons 1 2)) ; ==> 2
+
+Also some Lisps accept the "dotted pair" notation as a shorthand:
+    (1 . 2)
+
+
+And a list, composed of linked cons cells where `NIL` is the end sentinel:
+
+              cdr           cdr           cdr           cdr
+    +---+---+     +---+---+     +---+---+     +---+---+    
+    | O | O-|---->| O | O-|---->| O | O-|---->| o | O-|----> NIL
+    +-|-+---+     +---+---+     +---+---+     +---+---+    
+ car  |        car  |        car  |        car  |          
+     `'´           `'´           `'´           `'´         
+      20            "Lisp"        (1 2)        39 
+
+Using cons:
+    (cons 20 (cons "Lisp" (cons '(1 2) (cons 39 '()))))
+
+In cons dotted pair notation:
+    (20 . ("Lisp" . ('(1 2) . (39 . NIL))))
+
+Using quote:
+    '(20 "Lisp" '(1 2) 39)
+```
+
 ## Lisp languages in use
 - [Scheme](scheme.md): created by the hackers Guy Steele and Gerald Sussman in 1975 at the MIT. Has multiple implementations, Chez Scheme,
 Chicken Scheme, MIT/GNU Scheme, etc.