code: func ExistingDestination(dest string, fs FileSystem) Destination {
  	if fs == nil {
  		fs = NewFileSystem(filepath.Dir(dest), "")
  	}
  	return &fsDest{
  		fs:        fs,
  		dest:      dest,
  		atomic:    true, // note: txnFSDest unsets this, see Begin
  		openFiles: map[string]*os.File{},
  	}
  }
docs: // ExistingDestination returns an object that knows how to create files in an
  // existing directory in the file system.
  //
  // Will use a provided FileSystem object to operate on files if given, otherwise
  // uses a default one. If FileSystem is provided, dest must be in a subdirectory
  // of the given FileSystem root.
  //
  // Note that the returned object doesn't support transactional semantics, since
  // transactionally writing a bunch of files into an existing directory is hard.
  //
  // See NewDestination for writing files into a completely new directory. This
  // method supports transactions.