Non linear problem, linearized, how to solve?

[nelson -]

hi all,
  i'm resolving a non linear problem. Using a fixed point iteration, i have to solve my equation for the variable d = u_{new non linear step} - u_{old non linear step}. Can i do this in FiPy? After d is "little" in norm, i have to start a new timestep (I use a method similar to newton method). I'm planning to make some models involving Richards Equation (an equation used in groundwater modeling), i hope they will be included into fipy, if it will be possible :)

good day,
  nelson