[firedrake] .assign() with a function from a mixed function space

Dear all, I got confused with .assign() with a function from a mixed function space and I would appreciate some suggestions. The test code is in bitbucket.org/colinjcotter/slicemodels/branch/tidy revision f87e865. I expect a non-zero kinetic energy is printed out in line 54, but the result is still 0. To get an updated kinetic energy, I need line 57. Am I misunderstanding what .assign() and/or .split() do? All the best, Hiroe