code: def erosion(mapfile, dilated):
      
      ll = mappyfile.find(mapfile["layers"], "name", "line")
      ll["status"] = "OFF"
  
      pl = mappyfile.find(mapfile["layers"], "name", "polygon")
  
      # make a deep copy of the polygon layer in the Map
      # so any modification are made to this layer only
      pl2 = deepcopy(pl)
  
      pl2["name"] = "newpolygon"       
      mapfile["layers"].append(pl2)
  
      dilated = dilated.buffer(-0.3)
      pl2["features"][0]["wkt"] = dilated.wkt
  
      style = pl["classes"][0]["styles"][0]
      style["color"] = "#999999"
      style["outlinecolor"] = "#b2b2b2"
docs: We will continue to work with the modified Mapfile
      If we wanted to start from scratch we could simply reread it