Applying concentrated force

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?