code: func (source *source) addBaseProfiles(flagBase, flagDiffBase []*string) error {
  	base, diffBase := dropEmpty(flagBase), dropEmpty(flagDiffBase)
  	if len(base) > 0 && len(diffBase) > 0 {
  		return errors.New("-base and -diff_base flags cannot both be specified")
  	}
  
  	source.Base = base
  	if len(diffBase) > 0 {
  		source.Base, source.DiffBase = diffBase, true
  	}
  	return nil
  }
docs: // addBaseProfiles adds the list of base profiles or diff base profiles to
  // the source. This function will return an error if both base and diff base
  // profiles are specified.