code: func (e Embedded) AddCollection(rel Relation, r ResourceCollection) {
  	n := e[rel]
  	if n == nil {
  		//new embed
  		e[rel] = r
  		return
  	}
  
  	if nc, ok := n.([]*Resource); ok {
  		e[rel] = append(nc, r...)
  		return
  	}
  
  	if nr, ok := n.(*Resource); ok {
  		e[rel] = append([]*Resource{nr}, r...)
  	}
  
  }
docs: // AddCollection appends the resource into the list of embedded
  // resources with the specified relation.
  // r should be  a ResourceCollection