Here is a function which returns true if the argument is
 a list of atoms:

(defun ISLAT (L)

   (cond 

     ((null L) T)

     ((atom (car L))(ISLAT (cdr L)))

     (T F)

)))

====================================================

 Here is a function to check if atom A is a member of list L:

(defun member (A LAT)

    (cond

      ((null LAT) F)

      ((EQ (car LAT) A) T)

      (T (member A (cdr LAT)))

  )))


=========================================================

 Here is a function to remove an atom from a list:

(defun rember (A LAT)

   (cond

      ((null LAT) ())

      ((EQ (car LAT) A) (cdr LAT))

      (T (cons (car LAT)(rember A (cdr LAT))))

)))

========================================================

 A function to return a list of the first elements of each sublist 
in a list of lists: 

(defun firsts(L)

    (cond

    ((null L) ( ))
    (T (cons (car (car L)) (firsts (cdr L))))

)))

========================================================

 A function to insert NEW after the element OLD in the list L: 

(defun insertR (OLD NEW L)

   (cond

     ((null L) ())

     ((EQ (car L) OLD) (cons NEW (cdr L))  )

     (T (cons (car L) (insertR OLD NEW (cdr L))))

)))

=========================================================

  A function to substitute first occurence of OLD for NEW in list L: 

(defun substitute (OLD NEW L)

(cond

   (null L)())

   (EQ (car L) OLD) (cons NEW (cdr L)))

   (T (cons (car L) (substitute OLD NEW (cdr L))))

 )))

===================================================

 An example of a Do-loop: 

(defun dolength (L)

   (do (M L (cdr M)) (sum 0 (sum++)))

      ((NULL M) sum)

))

 OR 

(defun dolength(L)

     (do  ((M L)(sum 0)

         ((NULL M) sum)

         (setq M (cdr M))

         Setq sum (1+sum))))

===============================================

 Finding length of a list as a PROG program:

(defun proglength(L)

     (prog ((sum  0))

      again

      (cond ((null L)(return sum)))

      (setq sum (1+sum))

      (setq L (cdr L))

      (go again)))

================================================

A LIBRARY DATABASE 


(defun add_book(bookref title author publisher)

    (setf (get bookref 'title) title)

    (setf (get bookref 'author) author)

    (setf (get bookref 'publisher) publisher)

    (setq library (cons bookref library))

  bookref)

-------------------------

(defun retrieve_by (property value)

    (setq result nil)

    (do  ((L library (cdr L)))

          ((NULL L) result)

          (cond (EQ (get (car L) property) value)
                (cons (car L) result))))