def build_circuit(routers):
    """
        Builds a circuit consisting of exactly the routers specified,
        in order.  This issues a series of EXTENDCIRCUIT calls to Tor;
        the deferred returned from this is for the final
        EXTEND. FIXME: should return the Circuit instance, but
        currently returns final extend message 'EXTEND 1234' for
        example.
        """
