libMesh: A C++ Finite Element Library

Hi all,
  I have to use Libmesh to assemble an equation like this (i omitted a
more complicated term that now doesn't interest me :-))

U^{n} = U^{n-1}(1+(dt/eps))

where dt is the timestep and eps is a real problem parameter.

I have computed correctly u_old (as explained in the examples).... now
i try to assemble the RHS.

i have the following code but it doesn't work...
for (unsigned int qp = 0; qp < qrule.n_points(); qp++) {
 for (unsigned i = 0; i < n_dofs ; i++) {
   Fe(i) += JxW[qp]*(u_old*phi[i][qp] - (phi[i][qp] * ( dt / (eps);
				
i have tried also this but it doesn't works too :-(

for (unsigned int qp = 0; qp < qrule.n_points(); qp++) {
 for (unsigned i = 0; i < n_dofs ; i++) {
   Fe(i) += JxW[qp]*(u_old + (( dt / (eps) ) * u_old));

can anyone give me an help,
  nelson