{ A program written by Kevin Mitchell that uses
  the multitasking capabilities of xTurtle to
  draw a "tiling of the plane."  It is another
  example of the use of parallel processing.

  This tiling was designed by the Persian artist
  Mirza Akbar in the early 19th century.
  
  SUGGESTED:  Run this at "Fastest" speed with
  "Lockstep" turned on
}

sub rArrow
   penDown
   move(1,0)
   move(0,.5)
   move(1,1)
   move(.5,0)
   move(0,1)
   move(-1,0)
   move(0,-.5)
   move(-1,-1)
   move(-.5,0)
   move(0,-1)
end sub

sub lArrow
   penDown
   move(0,-1)
   move(.5,0)
   move(1,-1)
   move(0,-.5)
   move(1,0)
   move(0,1)
   move(-.5,0)
   move(-1,1)
   move(0,.5)
   move(-1,0)
end sub

sub upArrows
   fork(10)
   penUp
   moveTo(3*(forkNumber-5),0)
   fork(10)
   move(0,3*(forkNumber-5))
   rArrow
end sub

sub downArrows
   fork(10)
   penUp
   moveTo(3*(forkNumber-5)+1.5,1)
   fork(10)
   move(0,3*(forkNumber-5))
   lArrow
end sub

upArrows
downArrows