code: def _extract_keys(d: _LABEL_DICT, prefix: str = "") -> List[str]:
      """Nested key are described with slash: e.g. Animal.Bird -> Animal/Bird"""
      res: List[str] = []
      for k, v in d.items():
          res.append((prefix + k).upper())
          if isinstance(v, dict):
              res += _extract_keys(v, prefix + k + "/")
      return res
  
file_path: camphr/packages/camphr/camphr/ner_labels/utils.py
imports: import yaml
  from pathlib import Path
  from typing import IO, Dict, Iterable, List, Optional, Union
  import json
  from typing_extensions import Literal
label_desc: Use image node_id 5 for calling a global function with example usage: _extract_keys(d, prefix) and returns: res