How do I apply concentrated force in FF++?

So assume I have force

```
Vh(Th, P1);
Vh fx, fy, fz;
```

where `force = (fx, fy, fz)`

. Now In problem definition, I want to apply these forces to the nodes, for example in linear elasticity

```
solve lame([u, v], [uu, vv])
= int2d(Th)(
lambda * div(u, v) * div(uu, vv)
+ 2.*mu * ( epsilon(u,v)' * epsilon(uu,vv) )
)
- // Forces applied to nodes.
+ on(4, u=0, v=0)
;
```

Is there a way I can do that?