ALSA ttable and L-R

Hi fellow Picornauts,
I'd like to play around with false Stereo, so I'd want to transform L & R into L+R and L-R
My understanding of the ttable syntax is scant, I can see how to form L+R but then I need
L+(-R) don't know how to do that, anybody with a better grasp of ALSA ttables able to point me in the right direction
Thanks in advance
Strebor

Edit:
can it be as simple as?:

ttable.0.0 0.5
ttable.1.0 0.5
ttable.0.1 0.5
ttable.1.1 -0.5