code: func IDsFromEtc(rootPath, username, group string) (Resolver, error) {
  	return idsFromEtc{
  		rootPath: rootPath,
  		username: username,
  		group:    group,
  	}, nil
  }
docs: // IDsFromEtc returns a new UID/GID resolver by parsing etc/passwd, and etc/group
  // relative from the given rootPath looking for the given username, or group.
  // If username is empty string the etc/passwd lookup will be omitted.
  // If group is empty string the etc/group lookup will be omitted.