Zero the DRO

I want to put a button on the Glade panel which will zero out the DRO.  I don't want to HOME the axis as I want it to be good where it is.

On the panel, I am currently displaying the DRO value from position-fb:

HAL file:
net x-pos-fb    => pid.x.feedback
net x-pos-fb   <=  hm2_7i92.0.stepgen.02.position.fb
net x-pos-fb   => joint.0.motor-pos-fb

PostGUI HAL file:
net x-pos-fb    => gladevcp.X_DRO             This is what gets displayed

I am assuming that I need to add a HAL button to GladeVCP and then call a function in the handler python program.
Within that function, how do I write a 0 to this?