code: private static Optional<MutableSymbolTable> importSymbols(String filename) {
  
      URL resource;
      try {
        resource = Resources.getResource(filename);
      } catch (IllegalArgumentException e) {
        return Optional.absent();
      }
      return importSymbolsFrom(asCharSource(resource, Charsets.UTF_8));
    }
docs: Imports an openfst's symbols file
  
  @param filename the symbols' filename
  @return HashMap containing the impprted string-to-id mapping