[firedrake] trying to set up nonlinear problem

Dear Firedrakers, When I run the following script: https://gist.github.com/anonymous/32edd8c9c1614c1e2b67 I get: ufl.log.UFLException: All terms in form must have same rank. This is probably because I don't know how to set up nonlinear problems properly. Can anyone spot what is wrong? cheers --cjc