I solve time-dependent problem with the right hand side of
$$ L = \int_\Omega f(u) v dx $$
with nonlinear function $f(u)$ depending on the $u$ calculated on the previous step. Automatic assembling takes a lot of cpu time per iteration an Fenix manual recommends to make matrix operations using assembling only of mass and stiffness matrix. But how to compute matrix for L if function $f(u)$ is nonlinear and $u$ is a Function object?
