fxsubtract possible?

[Zimond]

In an older version of asphyre was a blendingeffect called fxsubtract. I used this effect to render holes into rendertargets by subtracting the color $ff000000 which made the alphachannel on the rendertarget go back to $00 (completly transparent)

With this trick i could create mirrors in my game engine which were defined by a rectangle and everything outside was subtracted.

I could re-enable the blending bug i mentioned in the unofficial release thread which actually exactly did that but that wouldn't work with DX11.