You can combine impermanence with death. I think when we separate them, we are looking more at one particular quality then the other as it applies in our life. So, if you separate it, you may want to go deeply into this type of characteristic, rather than combine it, and understand because there is death there is impermanence. So, when you don't want to go into more detail about reflecting on impermanence it is helpful to separate them. But certainly they can be combined as they are in the Five Reflections to understand impermanence through reflecting on death. I think it is a matter of if you go more deeply into one or the other, when it is helpful to separate them.