Wither Spawner

sense = peripheral.wrap("top")
mobType = 0

mobs = sense.getMobIds() -- find mobs

while true do
wither = false
for k, mobId in pairs(mobs) do --read information stored in mobs
  mob = sense.getMobData(mobId)
  mobType = mob.type
  print("mob: "..mobType) --print mob type
  print("at X: "..mob.position.x) --print mob coordinates
  print("at Y: "..mob.position.y)
  print("at Z: "..mob.position.z)
  if mobType == "Wither" and mob.position.x < 4 and mob.position.z < 4 then
    wither = true
    print("Wither Spawned")
    while wither do
      rs.setOutput("left",true)
      os.sleep(0.1)
      rs.setOutput("left",false)
      mobs = sense.getMobIds()
      witherPersists = false
      for k, mobId in pairs(mobs) do
        mob = sense.getMobData(mobId)
        if mob.type == "Wither" then
          witherPersists = true
        end
      end
      if witherPersists == false then
        wither = false
        rs.setOutput("front", true)
        os.sleep(0.5)
        rs.setOutput("front", false)
      end
    end
  end
end
if wither == false then
  rs.setOutput("back", true)
  os.sleep(3)
  rs.setOutput("back", false)
  rs.setOutput("right", true)
  os.sleep(1.3)
  rs.setOutput("right", false)
end
os.sleep(0.1)
end