; makes a world matrix
  (def makeworld (x)
    (= world (n-of x (n-of x nil))) ; world of nil
    (= merged (n-of x (n-of x nil))) ; layer of movers
    nil)


  ; print out the world, imposing a list of critters
  ; on top of it. The list itself assumes that its entries
  ; are tables which have a symbol and a position field
  (def show (lst)
    ; copy world into merged
    (for y 0 (- (len world) 1)
      (for x 0 (- (len world) 1)
        (= ((merged y) x) ((world y) x))))
    ; superimpose critters
    (each c lst
      (= ((merged (car (c "pos"))) (cadr (c "pos"))) (c "symbol")))
    ; plot world
    (each row merged
      (each pos row (if (is pos nil) (pr #\. #\space) (pr pos #\space)))
      (prn)))

  (= creature (table))
  (= creature2 (table))
  (= creature3 (table))
  (= (creature "pos") '(1 3))
  (= (creature2 "pos") '(10 3))
  (= (creature3 "pos") '(8 8))
  (= (creature "symbol") #\$)
  (= (creature2 "symbol") #\@)
  (= (creature3 "symbol") #\#)
  (makeworld 15)

  arc> (show (list creature creature2 creature3))
  . . . . . . . . . . . . . . . 
  . . . $ . . . . . . . . . . . 
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  . . . . . . . . # . . . . . . 
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  . . . @ . . . . . . . . . . . 
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  nil