docs: protected long[] joinLastDimensions(Subspace other) {
      if(this.dimensionality != other.dimensionality) {
        return null;
      }
      if(this.dimensionality == 1) {
        return BitsUtil.orI(BitsUtil.copy(this.dimensions), other.dimensions);
      }
      int last1 = BitsUtil.capacity(this.dimensions) - BitsUtil.numberOfLeadingZeros(this.dimensions) - 1;
      int last2 = BitsUtil.capacity(other.dimensions) - BitsUtil.numberOfLeadingZeros(other.dimensions) - 1;
      if(last2 < 0 || last1 >= last2) {
        return null;
      }
      long[] result = BitsUtil.orI(BitsUtil.copy(other.dimensions), this.dimensions);
      return BitsUtil.cardinality(result) == dimensionality + 1 ? result : null;
    }