code: func envFunc(env []string) func(string) (string, error) {
  	return func(s string) (string, error) {
  		for _, e := range env {
  			split := strings.SplitN(e, "=", 2)
  			k, v := split[0], split[1]
  			if k == s {
  				return v, nil
  			}
  		}
  		return os.Getenv(s), nil
  	}
  }
docs: // envFunc returns a function which checks the value of an environment variable.
  // Invokers can specify their own environment, which takes precedences over any
  // real environment variables