[Bob G]

I had a look at the alchemist's poison resistances, and tried to implement it for my ability, but since this ability is a custom ability, xIndex and xCount aren't valid fields. Instead, I tried using abValue as the argument to swap resistance for immunity:
<Pre-levels 1000>

If we're not active, just get out now
    doneif (tagis[Helper.SpcDisable] <> 0)

    ~ see if a quickfind has been created by another copy
    perform quickfind.setfocus

    ~ if we didn't find a redirection, we're the first copy of this ability to run this script, so we'll make ourselves the FirstCopy.
    if (state.isfocus = 0) then
      perform quickadd
      perform assign[Helper.FirstCopy]
      ~Add the base value of 2
      field[abValue].value += 2

    ~ otherwise, redirect hero.child[] to the FirstCopy, and make us an upgrade
    else
      perform focus.redirect
      perform assign[Helper.SpecUp]
      perform assign[Hide.Statblock]
      ~ add +2 to the focus's value
      focus.field[abValue].value += 2
      endif

<Post-levels 10000>

doneif (tagis[Helper.SpcDisable] <> 0)
    doneif (tagis[Helper.FirstCopy] = 0)

if (field[abValue].value <= 6) then
#situational[hero.child[svAll],signed(field[abValue].value) & " bonus on all saving throws against poison.",field[thingname].text]
else
      ~ If 4 copies are active, that's immunity
      if (field[abValue].value >= 8) then
        ~ We also want to change our special type from resistance to immunity
        perform delete[SpecType.Resist]
        perform assign[SpecType.Immune]

        ~ Since we're adding the immune to poison special, this ability
        ~ doesn't need to be shown once we're an immunity
        perform assign[Helper.SpecUp]
        endif
     endif

<Render 5000>

if (field[abValue].value <= 6) then
field[livename].text = field[thingname].text & " +" & field[abValue].value
else
field[livename].text = field[thingname].text & " Immunity to poison"
endif

Everything works until the 4th time the hero selects the ability. Resistance to Poison disappears, but immunity to poison doesn't replace it.