of L{IReactorFDSet} which only keeps track of which
    descriptors have been registered for reading and writing.

    This implementation isn't actually capable of determining readability or
    writeability and generates no events for the descriptors registered with
    it.

    @ivar _readers: A L{set} of L{IReadDescriptor} providers which the reactor
        is supposedly monitoring for read events.

    @ivar _writers: A L{set} of L{IWriteDescriptor} providers which the reactor
        is supposedly monitoring for write events.
    c