space embankment_space (const geo& omega, string approx) {
  space Xh (omega, approx, "vector");
  Xh.block("left");
  if (omega.dimension() >= 2) 
    Xh.block("bottom");
  if (omega.dimension() == 3) {
    Xh.block("right");
    Xh.block("back");
  }
  return Xh;
}
