$ clisp

<...snip...>

[1]> (load "quarto.l")
;; Loading file quarto.l ...
;;  Loading file lp.l ...
;;  Loaded file lp.l
;; Loaded file quarto.l
T
[2]> (task21--board-row-demo)
Animal playground...
d1=PIG d2=NIL d3=RAT d4=NIL
c1=NIL c2=EMU c3=FOX c4=OWL
b1=BAT b2=APE b3=AUK b4=YAK
a1=CAT a2=DOG a3=COW a4=NIL
Row d = (PIG NIL RAT NIL)
Row c = (NIL EMU FOX OWL)
Row b = (BAT APE AUK YAK)
Row a = (CAT DOG COW NIL)
NIL
[3]> (bye)
Bye.