local Bis in
    fun {Bis L3}
        case L3 of nil then
            nil
        else
            local Ans in
                if L3.2 == nil then
                    Ans = L3.1|nil
                else
                    Ans = L3.1|{Bis L3.2.2}
                end
                Ans
            end
        end
    end
    case L of nil then
        L1 = nil
        L2 = nil
    else
        L1 = {Bis L}
        L2 = {Bis L.2}
    end
end