entation for accessing the event loop.

    In this policy, each thread has its own event loop.  However, we
    only automatically create an event loop by default for the main
    thread; other threads by default have no event loop.

    Other policies may have different rules (e.g. a single global
    event loop, or automatically creating an event loop per thread, or
    using some other notion of context to which an event loop is
    associated).
    Nc