code: func (w *World) AddEntity(e Identifier) {
  	if w.sysIn == nil {
  		w.sysIn = make(map[reflect.Type]reflect.Type)
  	}
  	if w.sysEx == nil {
  		w.sysEx = make(map[reflect.Type]reflect.Type)
  	}
  	for _, system := range w.systems {
  		sys, ok := system.(SystemAddByInterfacer)
  		if !ok {
  			continue
  		}
  		if ex, not := w.sysEx[reflect.TypeOf(sys)]; not {
  			if reflect.TypeOf(e).Implements(ex) {
  				continue
  			}
  		}
  		if in, ok := w.sysIn[reflect.TypeOf(sys)]; ok {
  			if reflect.TypeOf(e).Implements(in) {
  				sys.AddByInterface(e)
  			}
  		}
  	}
  }
docs: // AddEntity adds the entity to all systems that have been added via
  // AddSystemInterface. If the system was added via AddSystem the entity will not be
  // added to it.