code: def prefix_token(self) -> str:
      return "<<"
  
file_path: H2O/h2o_hf/helm/src/helm/benchmark/window_services/mt_nlg_window_service.py
imports: from .gpt2_window_service import GPT2WindowService
  from .tokenizer_service import TokenizerService
label_desc: Use image node_id 4 for calling the MTNLGWindowService obj's underlying member method code with example usage: obj.prefix_token() and returns: str