anchor.diff:  ;; (= (/ pi 4) (@atan 2d0 2d0))
   ;; => T
   
  +;; (= (/ pi 4) (@atan 2 2))
  +;; => NIL
  +
   ;; also
   ;; (= (rationalize (/ pi 4d0)) (rationalize (@atan 2d0 2d0)))
  +;; = T
  +;;
  +;; as well:
  +;; (= (rationalize (/ pi 4)) (rationalize (@atan 2d0 2d0)))
  +;; => T ;; noting, pi is already a doulbe-float value
   ;;
   ;; although
   ;; (= (/ (rationalize pi) 4) (rationalize (@atan 2d0 2d0)))
  
positive.code: 
  (defconstant* %integer-instance-classes%
    (let ((c (compute-end-classes (find-class 'integer))))
      #+CCL (cons '(find-clas fixnum) c)
      #-CCL c))
  
  (defconstant* %float-instance-classes%
  
negative.code: ;; (= (/ (rational pi) 4) (rational (@atan 2d0 2d0)))
  ;; => T
  ;;
  
  ;;
  ;; thus illustrating some of the contrasting qualities of CL:RATIONAL 
  ;; and CL:RATIONALIZE - onto that simple wrapper for diadic ATAN
  