Tag Implication: [[ass_pillow]] -> [[head_on_butt]] ?

I'm not sure if this is right, but it seems to me that while head_on_butt doesn't always mean using the ass as a pillow, there's no way to have an ass_pillow where the head isn't on the butt.

I wonder if we need to keep the tags separate at all - what do others think?

They don't seem that different to me, if at all. I think one tag would be better.