code: function arrayDiff(a1, a2) {
      var o1={}, o2={}, diff=[], i, len, k;
      for (i=0, len=a1.length; i<len; i++) { o1[a1[i]] = true; }
      for (i=0, len=a2.length; i<len; i++) { o2[a2[i]] = true; }
      for (k in o1) { if (!(k in o2)) { diff.push(k); } }
      //for (k in o2) { if (!(k in o1)) { diff.push(k); } }
      return diff;
  }
docs: Utility diff function TODO: improve speed