;http://www.geocities.com/CollegePark/Lab/2276/codesf.htm#4
to square :size :ang :level
  if :level<1 [stop]
  lt :ang
  square :size -:ang :level-1 fd :size rt :ang
  square :size  :ang :level-1 fd :size
  square :size  :ang :level-1 rt :ang fd :size
  square :size -:ang :level-1 lt :ang
end

square 6 90 5