NetLogo编程——tick为什么无法运行

globals [number]
to setup 
  clear-all
  reset-ticks
  ask patches[set pcolor brown]
  set number 0
end
to turtleset
  set-default-shape turtles "circle"
  create-turtles 30[
    set color green
]
  ask turtles with [color = green][
    setxy random-xcor random-ycor
    set color red + 1
  ]
  set number number + 30
end
to go
  ask turtles [forward 1]
  if any? turtles with [distance myself < 1 and color = red + 1][set color blue]
  ask turtles with [color = blue][die]
  tick
end

这个代码为什么会出错，求解答！