def _fiss_agent_header(headers=None):
_set_session()<EOL>fiss_headers = {"<STR_LIT>" : FISS_USER_AGENT}<EOL>if headers is not None:<EOL><INDENT>fiss_headers.update(headers)<EOL><DEDENT>return fiss_headers<EOL>
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