Tracing Flattening Binary Tree

>(flattenbt '(cat (bat ape
                       bee)
                  (elf dog
                       fox)))

  1> (FLATTENBT (CAT (BAT APE BEE) (ELF DOG FOX)))
    2> (FLATTENBTB (CAT (BAT APE BEE) (ELF DOG FOX)) NIL)
      3> (FLATTENBTB (ELF DOG FOX) NIL)
        4> (FLATTENBTB FOX NIL)
        <4 (FLATTENBTB (FOX))
        4> (FLATTENBTB DOG (ELF FOX))
        <4 (FLATTENBTB (DOG ELF FOX))
      <3 (FLATTENBTB (DOG ELF FOX))
      3> (FLATTENBTB (BAT APE BEE) (CAT DOG ELF FOX))
        4> (FLATTENBTB BEE (CAT DOG ELF FOX))
        <4 (FLATTENBTB (BEE CAT DOG ELF FOX))
        4> (FLATTENBTB APE (BAT BEE CAT DOG ELF FOX))
        <4 (FLATTENBTB (APE BAT BEE CAT DOG ELF FOX))
      <3 (FLATTENBTB (APE BAT BEE CAT DOG ELF FOX))
    <2 (FLATTENBTB (APE BAT BEE CAT DOG ELF FOX))
  <1 (FLATTENBT (APE BAT BEE CAT DOG ELF FOX))
(APE BAT BEE CAT DOG ELF FOX)