code: func copyFileContents(srcFs, dstFs fs.Filesystem, src, dst string) (err error) {
  	in, err := srcFs.Open(src)
  	if err != nil {
  		return
  	}
  	defer in.Close()
  	out, err := dstFs.Create(dst)
  	if err != nil {
  		return
  	}
  	defer func() {
  		cerr := out.Close()
  		if err == nil {
  			err = cerr
  		}
  	}()
  	_, err = io.Copy(out, in)
  	return
  }
docs: // copyFileContents copies the contents of the file named src to the file named
  // by dst. The file will be created if it does not already exist. If the
  // destination file exists, all its contents will be replaced by the contents
  // of the source file.