Hair and Halos (just a tip)

Hi, Yesterday I stumbled across a problem when mixing ztransp hairs and Halos. I have these scene where the character is inside a volumetric light and everything else is black.
When I rendered the scene everything turned out fine, except for some white outlines that appeared at the areas where the hair, the character´s mesh, and the background overlapped.

I was going crazy, and thanks to that I discovered how powerful composite nodes are, I even discover by mistake how to make a sort of fake SSS with light going through the hairs. But that´s not the reason why I´m writing.

The thing is that nodes didn´t solve the problem. What did solve the problem was to put a simple plane geometry at the back of the scene. I asume that for some ALPHA stuff, the render was messed up when combining ztransp and halos with a ´´void´´ background.
So, by adding this plane at the back of the scene, there is now actual geometry in the background, and although everything looks the same (black background), the alpha of the scene changed and those white outlines between the background and the ztransp hair strands disappeared.