Hi!

I am solving a 3-dimensional linear elasticity problem. When declaring a solver class, you have to specify 3 displacement components and 6 components of the stress tensor as output. Is it possible to restrict ourselves to only a part of the stress tensor components (for example, 3)

At the moment I have something like this:

```
elasticity_net = self.arch.make_node(name='elasticity_net',
inputs=['x', 'y', 'z'],
outputs=['u', 'v', 'w',
'sigma_xx', 'sigma_yy', 'sigma_xy',
'sigma_zz', 'sigma_xz', 'sigma_yz'])
```

But I would like to see something like that:

```
elasticity_net = self.arch.make_node(name='elasticity_net',
inputs=['x', 'y', 'z'],
outputs=[ 'sigma_xx', 'sigma_yy', 'sigma_xy'])
```

An error occurs “could not unroll graph!”. What am I doing wrong?