code: func (c *Config) Set(key, value string) error {
  	c.Lock()
  	defer c.Unlock()
  
  	if _, found := c.data[key]; !found {
  		return ErrKey
  	}
  
  	separator := string(c.Separator())
  	replAt := []file.ReplacerAtLine{
  		{key + separator, separator + ".*", separator + value},
  	}
  
  	if err := file.ReplaceAtLine(c.filename, replAt); err != nil {
  		return err
  	}
  	c.data[key] = value
  	return nil
  }
docs: // Set writes a new value for key.