code: func (c *ArgCommand) Expand(expander SingleWordExpander) error {
  	p, err := expander(c.Key)
  	if err != nil {
  		return err
  	}
  	c.Key = p
  	if c.Value != nil {
  		p, err = expander(*c.Value)
  		if err != nil {
  			return err
  		}
  		c.Value = &p
  	}
  	return nil
  }
docs: // Expand variables