code: func (lsel internalSelector) Add(reqs ...Requirement) Selector {
  	var sel internalSelector
  	for ix := range lsel {
  		sel = append(sel, lsel[ix])
  	}
  	for _, r := range reqs {
  		sel = append(sel, r)
  	}
  	sort.Sort(ByKey(sel))
  	return sel
  }
docs: // Add adds requirements to the selector. It copies the current selector returning a new one