MathObj.PowerSeries2

In order to handle both variables equivalently we maintain a list of coefficients for terms of the same total degree. That is

eval [[a], [b,c], [d,e,f]] (x,y) ==
   a + b*x+c*y + d*x^2+e*x*y+f*y^2

Although the sub-lists are always finite and thus are more like polynomials than power series, division and square root computation are easier to implement for power series.