Has the Points/MathPts node been fully implemented yet?

I ask because it seems to me that variables B & C have not been implemented in that way that variable A has been.
EDIT: Or perhaps the use of variables has not been implemented for X and Z?

I was trying to make a simple nod to produce Brian's HyperbolicParaboloid2:
_FxGraph3D a*(u+v);b*(u-v);u*v;-1;1;-1;1;50;50;1.618;1;0;UV see post 6447.73 and couldn't get it to work using variables A, B & C in Points/MathPts as below.



when I deleted variables B & C (shown below) it worked



Here's the working nod: