code: func (e Embedded) Add(rel Relation, r *Resource) {
  	n := e[rel]
  	if n == nil {
  		//new embed
  		e[rel] = r
  		return
  	}
  
  	if nec, ok := n.([]*Resource); ok {
  		e[rel] = append(nec, r)
  		return
  	}
  
  	if nee, ok := n.(*Resource); ok {
  		e[rel] = append([]*Resource{nee}, r)
  		return
  	}
  
  	//something went wrong.. replace what is there with what is new
  	e[rel] = []*Resource{r}
  }
docs: // Add appends the resource into the list of embedded
  // resources with the specified relation.
  // r should be a Resource