1.  
  2. (defun erase (l)
  3.   (cond ((endp l) nil)
  4.         ((= (cdr l) nil) nil)
  5.         (t
  6.          (cons (car l) (erase (cdr l))))))
  7.  
  8. ;; prefix : truelistp -> truelistp
  9. ;; produces a list of its prefixes
  10. (defun prefix (l)
  11.   (cond ((not(true-listp l)) nil)
  12.         ((endp l) '())
  13.         (t
  14.          (cons l (prefix (erase l))))))