code: func (uw *UnitWriter) Activate(unit, wantPath string) {
  	if uw.err != nil {
  		return
  	}
  
  	if err := os.Symlink(path.Join("..", unit), wantPath); err != nil && !os.IsExist(err) {
  		uw.err = errwrap.Wrap(errors.New("failed to link service want"), err)
  	}
  }
docs: // Activate actives the given unit in the given wantPath.