struct Tower

    ...

    private static method DistributeExpFilterFunc takes nothing returns nothing
        local thistype t = GetUnitUserData(GetFilterUnit())
        if IsFilterUnitTower() and t.isReady and t.data.hasexp and t.selfupgrade then
            call t.addExp(TransmittedExpAmount)
        endif
    endmethod
    
    private static boolexpr DistributeExpFilter
    private static integer TransmittedExpAmount
    
    static method distributeExp takes Minion m, thistype killer returns nothing
        local real x = GetUnitX(m.minion)
        local real y = GetUnitY(m.minion)
        set TransmittedExpAmount = m.data.towexpgain
        // Add to killer if it has experience, is upgradable, but is out of range.
        if killer.data.hasexp and killer.selfupgrade and not IsUnitInRangeXY(killer.tower, x, y, ExpRange) then
            call killer.addExp(TransmittedExpAmount)
        endif
        call GroupEnumUnitsInRange(bj_lastCreatedGroup, x, y, ExpRange, DistributeExpFilter)
    endmethod
    
    ...

    private static method UpgradeTowerFilterFunc takes nothing returns boolean
        local thistype t = GetUnitUserData(GetFilterUnit())
        return IsFilterUnitTower() and t.isReady and not t.data.hasexp and t.selfupgrade and t.owner.isGoldEnough(t.ToNextLevel)
    endmethod
    
    private static boolexpr UpgradeTowerFilter
    
    static method upgradeTower takes thistype this returns nothing
        local thistype array selected
        local thistype temp1
        local thistype temp2
        local integer n = -1
        local integer i = -1
        local group g = CreateGroup()
        local unit u
        call GroupEnumUnitsSelected(g, this.owner.user, UpgradeTowerFilter)
        // Sort selected towers from tower with the lowest cost to the highest.
        loop
            set u = FirstOfGroup(g)
            exitwhen u == null
            call GroupRemoveUnit(g, u)
            set temp1 = GetUnitUserData(u)
            set n = n + 1
            set selected[n] = temp1
            loop
                exitwhen i < 0 or selected[i].ToNextLevel <= temp1.ToNextLevel
                set temp2 = selected[i]
                set selected[i] = temp1
                set selected[i + 1] = temp2
                set i = i - 1
            endloop
            set i = n
        endloop
        call DestroyGroup(g)
        set g = null
        if n > -1 then
            // Upgrade towers starting from towers with the highest cost.
            loop
                set temp1 = selected[n]
                exitwhen not this.owner.spendGoldNoMessage(temp1.ToNextLevel)
                call temp1.incLevel()
                set n = n - 1
                exitwhen n == -1
            endloop
        else
            // Raise "Not enough Gold" error.
            call this.owner.messageNotEnoughGold()
        endif
    endmethod
    
    private static method onInit takes nothing returns nothing
        ...
        set DistributeExpFilter = Filter(function thistype.DistributeExpFilterFunc)
        set UpgradeTowerFilter = Filter(function thistype.UpgradeTowerFilterFunc)
    endmethod
endstruct