constexpr int i0 = (ORDER + 1) / 2;

    const int i0 = (ORDER + 1) / 2 - off;

    // for (int i = 0; i < ORDER + 1; ++i)
    //   y += cs[i] * crseptr[(i - i0) * di];

    for (int i = 0; i < i0; ++i)
      y += cs[i] * (crseptr[(i - i0) * di] + crseptr[(i0 - 1 - i) * di]);

    for (int i = 0; i < (ORDER + 1) / 2; ++i) {
      const int i1 = ORDER - i;
#ifdef CCTK_DEBUG
      assert(cs[i1] == cs[i]);
#endif
      y += cs[i] * (crseptr[(i - i0) * di] + crseptr[(i1 - i0) * di]);
    }

#endif
#ifdef CCTK_DEBUG
    T y1 = 0;
    for (int i = 0; i < ORDER + 1; ++i)
      y1 += cs[i] * crseptr[(i - i0) * di];
    assert(!CCTK_isnan(y1));
    // Don't check for equality; there can be round-off errors
    // assert(y1 == y);