Dirichlet boundary conditions which depend on solution

Question

Hey there,

How do I implement Dirichlet boundary conditions which are functions of the solution variables.

For example, if my PDE has a solution (u,v) and my B.C. is:

$\begin{equation}
v=u\quad \mathrm{on}\quad \Gamma
\end{equation}$

Then my code would look something like this:

#
v_bc = u
bc_v = DirichletBC(W.sub(1), v_bc, gamma)
#

But this gives me errors like:

[0]PETSC ERROR: Arguments are incompatible!
[0]PETSC ERROR: Local size 10201 not compatible with block size 5!

Any help would be much appreciated!