信息素的开发结束了它的变异

Question

这是我们的程序，但是我们有一个问题，信息素永远不会为0，我们需要它为0应该做什么？

 to LayPheromone 
if (pheromone < maxPheroPerSite-Forage)
[set pheromone (pheromone + amtPheroLayForage)]       
end 
 to UpdatePatches 

 ask patches [ if (pheromone <0)               
 [set pheromone 0] 
  if (pheromone > 0) 
  [ set pheromone (pheromone - pheromone * evaporation-rate) 
 ifelse 
 showPhero?
  [ set pcolor scale-color yellow pheromone 0 (maxPheroPerSite-Forage)] 
  if pheromone > = maxPheroPerSite-Return [ set pcolor black ] ] ]

  to LayPheromone 
 if (pheromone < maxPheroPerSite-Forage)
[set pheromone (pheromone + amtPheroLay-Forage)] 
end
to UpdatePatches
ask patch nestX nestY [ set pcolor pink ] 
 ask patches [ if (pheromone < 0) [set pheromone 0] 
 if (pheromone > 0)
[ set pheromone (pheromone - pheromone * evaporation-rate) 
 ifelse showPhero?
 [ set pcolor scale-color yellow pheromone 0 (maxPheroPerSite-Forage)] 
 if pheromone > = maxPheroPerSite-Return [ set pcolor black ] ] ] 
  end