code: func (d *SourceFilesystem) MakePathRelative(filename string) string {
  	for _, currentPath := range d.Dirnames {
  		if strings.HasPrefix(filename, currentPath) {
  			return strings.TrimPrefix(filename, currentPath)
  		}
  	}
  	return ""
  }
docs: // MakePathRelative creates a relative path from the given filename.
  // It will return an empty string if the filename is not a member of this filesystem.