docs: function(diff) {
            if (!diff) return this.hasChanged() ? clone(this.changed) : false;
            var old = this._changing ? this._previousAttributes : this.attributes;
            var changed = {};
            for (var attr in diff) {
              var val = diff[attr];
              if (isEqual(old[attr], val)) continue;
              changed[attr] = val;
            }
            return !isEmptyObject(changed) ? changed : false;
          }