code: public static long[] xorI(long[] v, long[] o) {
      assert (o.length <= v.length) : "Bit set sizes do not agree.";
      for(int i = 0; i < o.length; i++) {
        v[i] ^= o[i];
      }
      return v;
    }
docs: XOR o onto v in-place, i.e. v ^= o
  
  @param v Primary object
  @param o data to xor
  @return v