function step()
   function get(x,y)
      local r={}
      r[0]=0
      if tpt.get_property("type",x,y)==1 then
         if tpt.get_property("tmp",x,y)~=1 then
            r[0]=r[0]+1
            r[r[0]][0]=tpt.get_property("vx",x,y)
            r[r[0]][1]=tpt.get_property("vy",x,y)
            tpt.set_property("tmp",1,x,y)
            local l={}
            l=get(x,y+1)
            for i=1,l[0] do
               r[0]=r[0]+1
               r[r[0]][0]=l[i][0]
               r[r[0]][1]=l[i][1]
            end
            l=get(x,y-1)
            for i=1,l[0] do
               r[0]=r[0]+1
               r[r[0]][0]=l[i][0]
               r[r[0]][1]=l[i][1]
            end
            l=get(x+1,y)
            for i=1,l[0] do
               r[0]=r[0]+1
               r[r[0]][0]=l[i][0]
               r[r[0]][1]=l[i][1]
            end
            l=get(x-1,y)
            for i=1,l[0] do
               r[0]=r[0]+1
               r[r[0]][0]=l[i][0]
               r[r[0]][1]=l[i][1]
            end
         end
      end
      return r
   end

   function set(x,y,vx,vy)
      if tpt.get_property("type",x,y)==1 then
         if tpt.get_property("tmp",x,y)==1 then
            tpt.set_property("vx",vx,x,y)
            tpt.set_property("vy",vy,x,y)
            tpt.set_property("tmp",0,x,y)
            set(x,y,vx+1,vy)
            set(x,y,vx-1,vy)
            set(x,y,vx,vy+1)
            set(x,y,vx,vy-1)
         end
      end
   end

   local r=get(tpt.get_property("x",id),tpt.get_property("y",id))
   local sx=0
   local sy=0
   for i=1,r[0] do
      sx=sx+r[i][0]
      sy=sy+r[i][1]
   end
   set(tpt.get_property("x",id),tpt.get_property("y",id),sx/r[0],sy/r[0])
end

function click(x,y,b,e)
   if tpt.get_property("type",x,y)==1 then
      tpt.delete(x,y)
      id=tpt.create(x,y,1)
      tpt.message_box(id)
      tpt.register_step(step)
      tpt.unregister_mouseevent(click)
   end
end

tpt.register_mouseevent(click)