proucedure of pheromone end its variation 2

问题

this is our procedure but we have a problem , the pheromone never be 0 and we need it to be 0 what should 'i do ?

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)]

      [ set pcolor black ]
    ]
] `

来源：https://stackoverflow.com/questions/23257442/proucedure-of-pheromone-end-its-variation-2