code: function(models, at){
          var currentLen = this.length;
  
          models = Array.from(models);
  
          this.act(function(){
              var len = models.length,
                  i = 0;
  
              while(len--){
                  this._add(models[i++], at);
              }
          });
  
          // Signal change when the collection of models change
          if (!this.isActive() && this._changed) {
              this.signalChange();
  
              this.resetChange();
          }
  
          return this;
      }
docs: Add a model or models
  @param {Class || Array} A single Model instance or an array of Model instances
  @param {Number} at The index at which the model should be inserted
  @return {Class} Collection Instance
  
  @example
  collectionInstance.add(model, at);
  collectionInstance.add([model, model], at);