code: def _fiss_agent_header(headers=None):
      
      _set_session()
  
      fiss_headers = {"User-Agent" : FISS_USER_AGENT}
      if headers is not None:
          fiss_headers.update(headers)
      return fiss_headers
docs: Return request headers for fiss.
          Inserts FISS as the User-Agent.
          Initializes __SESSION if it hasn't been set.
  
      Args:
          headers (dict): Include additional headers as key-value pairs