code: function (key, instance) {
              if (key instanceof View) {
                  instance = key;
                  key = undefined;
              }
              this._addInstance(key, instance);
              return this;
          }
docs: Add a subview instance. If is has a config,
  the instance will be associated with that
  config. If the config specifies that the
  view is a singleton an a view for that
  key already exists, it will not be added.
  @param {string|number} key
  @param {Backbone.View} instance
   
  Add a subview instance, without a key.
  Since it doesn't have a key, it will
  simply be added to the subViews array.
  @param {Backbone.View} instance