void print_2d_cell(FILE *wr, interval_t cell[])
  { fprintf(wr, 
      "[%+8.5f _ %+8.5f]×[%+8.5f _ %+8.5f]\n",
      LO(cell[0]), HI(cell[0]), 
      LO(cell[1]), HI(cell[1])
    );
  }