code: func (s SourceFilesystems) MakeStaticPathRelative(filename string) string {
  	for _, staticFs := range s.Static {
  		rel := staticFs.MakePathRelative(filename)
  		if rel != "" {
  			return rel
  		}
  	}
  	return ""
  }
docs: // MakeStaticPathRelative makes an absolute static filename into a relative one.
  // It will return an empty string if the filename is not a member of a static filesystem.