code: protected void appendCopy(RoaringArray sa, int index) {
      extendArray(1);
      this.keys[this.size] = sa.keys[index];
      this.values[this.size] = sa.values[index].clone();
      this.size++;
    }
docs: Append copy of the one value from another array
  
  @param sa other array
  @param index index in the other array