docs: void rowColUpdate(final int rowCol) {
      final int col = rowCol & 63;
      if (col < fiCol) { return; } // important speed optimization
      final long c = numCoupons;
      if (c == 0) { promoteEmptyToSparse(this); }
      final long k = 1L << lgK;
      if ((c << 5) < (3L * k)) { updateSparse(this, rowCol); }
      else { updateWindowed(this, rowCol); }
    }