sbool zipFileSnippetToLazyTextMap_subfolder_debug;

// keeps file open until you cleanUp() the map
static SS zipFileSnippetToLazyTextMap_subfolder(S snippetID, S folderName) ctex {
  if (empty(folderName)) ret zipFileSnippetToLazyTextMap(snippetID);
  fS prefix = addSlash(folderName);
  final ZipFile zip = new ZipFile(loadLibrary(snippetID));
  O f = func(S name) {
    if (zipFileSnippetToLazyTextMap_subfolder_debug)
      print("subfolder get " + prefix+name);
    ret loadTextFileFromZipFile(zip, prefix + name);
  };
  ret autoCleanUp(new LazyValueTreeMap(startingWith_dropPrefix(listFilesInZip(zip), prefix), f) {
    public void finalize() ctex {
      cleanMeUp();
    }
    
    void cleanMeUp() {
      if (zipFileSnippetToLazyTextMap_subfolder_debug)
        print("subfolder cleanup");
      pcall-silent { zip.close(); }
    }
  });
}

Began life as a copy of #1008856