GOTO

#<z> = 0
N1000
  G00 X0 Y0 Z#<z>
  G01 X20 Y0
  G01 X20 Y20
  G01 X0 Y20
  G01 X0 Y0
  #<z> = [#<z>+1]
  IF [#<z> GT 30] THEN GOTO 2000
  IF [#<z> GT 10] THEN #<z> = [#<z>+1]
  IF [#<z> GT 20] THEN #<z> = [#<z>+2]
  GOTO 1000
N2000
M2