code: function (old, current) {
              var current = current.slice(0),
                  old = old.slice(0);
  
              // remove intersection from each array
              for (var i = 0; i < current.length; i++) {
                  for (var j = 0; j < old.length; j++) {
                      if (equal(this.opts.id(current[i]), this.opts.id(old[j]))) {
                          current.splice(i, 1);
                          if(i>0){
                          	i--;
                          }
                          old.splice(j, 1);
                          j--;
                      }
                  }
              }
  
              return {added: current, removed: old};
          }
docs: multi