def supersede_ceo(company_id: str, new_ceo_id: str, effective_date: str):
    current = graph.find_edge(
        source=company_id,
        relation="CEO",
        valid_to=None
    )
    if current:
        graph.update_edge(current.id, valid_to=effective_date)

    graph.add_edge(
        source=company_id,
        relation="CEO",
        target=new_ceo_id,
        valid_from=effective_date,
        valid_to=None,
    )