(interp (app (fun 'f (app (id 'f) (num 8)))  ; {{fun {f} {f 8}}
              (fun 'y (add (id 'y) (num 4)))) ;  {fun {y} {+ y 4}}}
         (mtSub))
...
  (interp (fun 'f (app (id 'f) (num 8)))
          (mtSub))
  returns (closureV 'f (app (id 'f) (num 8)) (mtSub))
...
  (interp (fun 'y (add (id 'y) (num 4)))
          (mtSub))
  returns (closureV 'y (add (id 'y) (num 4)) (mtSub))
...
(interp (app (id 'f) (num 8))
        ;; ds1 = 
        (aSub 'f
              (closureV 'y (add (id 'y) (num 4)) (mtSub))
              (mtSub)))
....
   (interp (id 'f) ds1)
   returns (closureV 'y (add (id 'y) (num 4)) (mtSub))
...
   (interp (num 8) ds1)
   returns (numV 8)
...
(interp (add (id 'y) (num 4))
        ;; ds2 = 
        (aSub 'y
              (numV 8)
              (mtSub)))
....
  (interp (id 'y) ds2)
  returns (numV 8)
  ...
  (interp (num 4) ds2)
  return (numV 4)

...returns (numV 12)