docs: protected void append(RoaringArray sa, int startingIndex, int end) {
      extendArray(end - startingIndex);
      for (int i = startingIndex; i < end; ++i) {
        this.keys[this.size] = sa.keys[i];
        this.values[this.size] = sa.values[i];
        this.size++;
      }
    }