code: function (dt) {
              _.each(
                  systems,
                  function (system) {
                      if (_.has(system, 'update')) {
                          _.each(
                              entities,
                              function (entity) {
                                  if (!_.isUndefined(entity)) {
                                      _.each(system.update, function (func, id) {
                                          if (entity.has(id)){
                                              func(entity, dt);
                                          }
                                      });
                                  }
                              }
                          );
                      }
                  }
              );
          }
docs: Call all registered system's update function, passing a timestep.