[firedrake] memory leak?

Dear all, We are getting a memory leak in our slice model solver and we can't work out where we are making it. The time loop pretty much just consists of some solver calls and Function += calls. To see our test, please check out https://bitbucket.org/colinjcotter/slicemodels and run test_linear.py. We are observing a slow and steady increase in memory usage. cheers Colin and Hiroe