https://fenicsproject.org/olddocs/dolfin/1.5.0/python/demo/documented/mixed-poisson/python/documentation.html

In 9.1

We are now ready to define the variational forms a and L. Since, u0=0u0=0 in this example, the boundary term on the right-hand side vanishes.

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Define variational form a = (dot(sigma, tau) + div(tau)*u + div(sigma)*v)**dx L = - f*vdx

*dx L = - f*v

It only remains to prescribe the boundary condition for the flux. Essential boundary conditions are specified through the class `DirichletBC`

which takes three arguments: the function space the boundary condition is supposed to be applied to, the data for the boundary condition, and the relevant part of the boundary.

**Is it possible to get into details of this? Why did you combine these two?**

**Is it to possible to do it separately?**