procedure main(av)

  w1 := open(av[1],"g","canvas=hidden","image="||av[1]) |
      stop("can't open window or image file argument")
  wid := WAttrib(w1, "width")
  ht := WAttrib(w1, "height")
  w2 := open(av[1],"g","size="||(wid*2)||","||(ht*2))
  CopyArea(w1,w2,0,0,wid,ht,0,0)
  CopyArea(w1,w2,0,0,wid,ht,wid,ht)
  CopyArea(w1,w2,0,0,wid,ht,0,ht)
  CopyArea(w1,w2,0,0,wid,ht,wid,0)

  WAttrib(w2,"label=Tilinator: is it good enough already?")
  if Event(w2) === "y" then exit(0)

  x := y := 0
  wid2 := wid * 2
  ht2 := ht * 2
  every p := Pixel(w1) do {
     x +:= 1
     if x = wid then { x := 0; y +:= 1 }
     Fg(w2, p)
     mirx := wid2 - x - 1
     miry := ht2 - y - 1 
     DrawPoint(w2, mirx, miry, mirx, y, x, miry)
     }

  WAttrib(w2,"label=Tilinator: now is it good enough?")
  if Event(w2) === "y" then {
      WriteImage(w2,"tile-" || av[1], 0, 0)
      exit(0)
  }

  # go off and do a more serious mathematical tiling

end