This implements the fourth heuristic of the Maple Paper. The main aim
of this heuristic is to solve the partial differential equation
chi.diff(x) + h*chi.diff(y) - hy*chi = 0 for chi and set values
for eta and xi such that eta = xi*h + chi

It assumes chi to be a bivariate polynomial in x and y

Implemented algo to find xi and eta

Corrected minor test failure on heuristic 3