为什么plabel会替换此netlogo模型中的先前plabel以显示补丁和乌龟协调？

Question

使用help of @JenB我编写了一个代码来帮助理解forward 1，补丁和乌龟协调。代码附在最后。

但是，我发现了一个错误：我在点击setup然后点击test几次后，构建了两个按钮setup和test，我们会注意到一些龟1号的脚will将取代以前的脚，，这根本不是预期的，不应该发生;然而，乌龟0似乎没有这样的问题。

to setup
  clear-all
  create-ordered-turtles 2 
  [ 
    ifelse (who mod 2) = 0 
    [
      set heading heading + 15 
    ]
    [
      set heading 135
    ]
    pen-down
;    forward 1
  ]

end

to test
  ask turtles [   ;; for each turtle
    fd 1

    ;; get patch corrdination 
    let patch-cor (word "px" pxcor ":" "py" pycor)
    ;; get turtle corrdination
    let turtle-cor (word "x" (precision xcor 2) ":" "y" (precision ycor 2))
    ;; get distance between current and previous turtle corrdinations

    ;; get distance between current and origin turtle corrdinations
    let distanceTurtle precision (distancexy 0 0) 1

    let patch-color color                               ;; get turtle's color


    set label patch-cor                                 ;; let this turtle's label show current patch-corrdination
    set label-color patch-color

    ask patch-at -1 0                                   ;; focus on patch 1 unit left to where this turtle is on
    [
      set plabel patch-cor                              ;;  let this patch label show turtle's patch corrdination
      set plabel-color patch-color
    ]

    ask patch-at 4 0                                    ;; focus on patch 4 unit right to where this turtle is on
    [
      set plabel turtle-cor                             ;;  let this patch label show turtle's own corrdination
      set plabel-color patch-color
    ]

    ask patch-at 5 0                                    ;; focus on patch 5 unit right to where this turtle is on
    [
      set plabel distanceTurtle               ;;  let this patch label show distance between current turtle and origin
      set plabel-color patch-color
    ]
  ]

end