code: func (c *CopyCommand) Expand(expander SingleWordExpander) error {
  	expandedChown, err := expander(c.Chown)
  	if err != nil {
  		return err
  	}
  	c.Chown = expandedChown
  	return expandSliceInPlace(c.SourcesAndDest, expander)
  }
docs: // Expand variables