code: func (c *ConfigFile) MustValueSet(section, key string, defaultVal ...string) (string, bool) {
  	val, err := c.GetValue(section, key)
  	if len(defaultVal) > 0 && (err != nil || len(val) == 0) {
  		c.SetValue(section, key, defaultVal[0])
  		return defaultVal[0], true
  	}
  	return val, false
  }
docs: // MustValue always returns value without error,
  // It returns empty string if error occurs, or the default value if given,
  // and a bool value indicates whether default value is returned.