code: func (b *Builder) Del(ns ...string) *Builder {
  	for _, n := range ns {
  		for i, a := range b.add {
  			if a.Name == n {
  				b.add = append(b.add[:i], b.add[i+1:]...)
  			}
  		}
  		b.del = append(b.del, n)
  	}
  	return b
  }
docs: // Del deletes the label of the given name.