code: func (r *Registry) PutEntity(parent mo.Entity, item mo.Entity) mo.Entity {
  	e := item.Entity()
  
  	if parent != nil {
  		e.Parent = &parent.Entity().Self
  	}
  
  	r.Put(item)
  
  	return item
  }
docs: // PutEntity sets item.Parent to that of parent.Self before adding item to the Registry.