code: function (entity, just) {
              _.each(
                  systems,
                  function (system) {
                      if (_.has(system, 'destroy')) {
                          _.each(system.destroy, function (func, id) {
                              if (entity.has(id)){
                                  // Only remove from tracking systems.
                                  if (!_.isUndefined(just)) {
                                      if (id !== just) {
                                          return;
                                      }
                                  }
                                  // Perform the remove.
                                  func(entity);
                              }
                          });
                      }
                  }
              );
          }
docs: Destroys an entity in all registered systems. If 'just' is given, just systems that deal with that component destroy the entity rather than all.