code: func MultiEnvDefaultFunc(ks []string, dv interface{}) SchemaDefaultFunc {
  	return func() (interface{}, error) {
  		for _, k := range ks {
  			if v := os.Getenv(k); v != "" {
  				return v, nil
  			}
  		}
  		return dv, nil
  	}
  }
docs: // MultiEnvDefaultFunc is a helper function that returns the value of the first
  // environment variable in the given list that returns a non-empty value. If
  // none of the environment variables return a value, the default value is
  // returned.