struct u_exact : field_functor<u_exact,Float> {
  Float operator() (const point& x) const { 
    return cos(pi*x[0])*cos(pi*x[1])*cos(pi*x[2]); }
  u_exact(size_t d1) : d(d1), pi(acos(Float(-1.0))) {}
  size_t d; Float pi;
};
