// 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) { loadTextFileFromZipFile(zip, prefix + name) };
  ret new LazyValueTreeMap(startingWith_dropPrefix(listFilesInZip(zip), prefix), f) {
    void cleanMeUp() ctex {
      zip.close();
    }
  };
}