[yt-users] cut_region: can I make an expression dimensionless?

[Stuart Levy]

Hello yt people,

I'm hoping to write a cut_region expression that (a) depends on position 
and (b) does it in a nonlinear way - I'd like to do a sort of 
paraboloidal cut, like

     ds.cut_region(dd, "obj['z'] - (obj['x']**2 + obj['y']**2)/2.0 < 0.78")

But this runs afoul of the unit-checking - obj['z'] doesn't have the 
same units as obj['x']**2.

Somehow I either need to cast all the obj[] terms to be dimensionless 
("trust me, I promise it's right"), or else give dimensions of length to 
the constants.   Should there be a way to do either one?