void dirichlet (const field& lh, field& uh) {
  const space& Xh = lh.get_space();
  trial u (Xh); test v (Xh);
  form a = integrate (dot(grad(u),grad(v)));
  solver sa (a.uu());
  uh.set_u() = sa.solve (lh.u() - a.ub()*uh.b());
}
